WIP: Panel Transpose Cleanup
Progress towards #25632 

Prefacing this PR with the statement that this is pretty wonky, but I would consider that largely an artifact of cleaning up the existing code base. 

Right now `transpose` is defined in `NDFrame` but is only used by `DataFrame` and `Panel`, as `Series` objects end up using a separate `transpose` defined in `IndexOpsMixin`. 

There are duplicate calls to `validate_transpose*` with various signatures that aren't well defined, and everything is passed rather ambiguously via args and kwargs which makes validation rather difficult.

Existing validation is arguably rough. Here's a sample call and response on master:

```python-traceback
>>> df = pd.DataFrame(np.ones((2, 2)))
>>> df.transpose('foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/williamayd/clones/pandas/pandas/core/frame.py", line 2643, in transpose
    nv.validate_transpose(args, dict())
  File "/Users/williamayd/clones/pandas/pandas/compat/numpy/function.py", line 55, in __call__
    self.defaults)
  File "/Users/williamayd/clones/pandas/pandas/util/_validators.py", line 218, in validate_args_and_kwargs
    validate_kwargs(fname, kwargs, compat_args)
  File "/Users/williamayd/clones/pandas/pandas/util/_validators.py", line 157, in validate_kwargs
    _check_for_default_values(fname, kwds, compat_args)
  File "/Users/williamayd/clones/pandas/pandas/util/_validators.py", line 69, in _check_for_default_values
    format(fname=fname, arg=key)))
ValueError: the 'axes' parameter is not supported in the pandas implementation of transpose()
```

As you can see the error message mentions the 'axes' parameter, but that isn't part of the signature of `transpose` and is instead ambiguously defined based off of args.

What I've done here is removed the Panel implementation and tried to make the `NDFrame` implementation more explicit. Because it's only used by `DataFrame` however, an alternate and arguably preferable implementation would be to raise `NotImplementedError` in `NDFrame` and handle everything in the `DataFrame` implementation once Panel is gone
Should also mention that this is somewhat of an "optional" cleanup. What brought me here was seeing `Panel` appear in the comment of `validate_transpose_for_generic` and then tracing that back to see that there were potentially duplicative calls to `validate_transpose` for DataFrame, whereas Panel solely relied on the former

At the end of the day Panel could simply be removed and no change, but I figure this is worth cleaning up to simplify is possible @WillAyd : I understand what you're trying to do, but the motivation for having those "kwargs" and "args" in the signature was for `numpy` compatibility, which not ideal for us maintainers, was something that came up as an issue for users, hence why we implemented it.

If this is just cleanup, I would just make sure we can separate out this method so that we can cleanly take it out for `Panel` at the end of the day.  Don't modify the signature at this point. > the motivation for having those "kwargs" and "args" in the signature was for `numpy` compatibility, which not ideal for us maintainers, was something that came up as an issue for users

Do you know if there was an issue attached to that? Without knowing the backstory just looking at the current implementation it seems like there's a lot of unnecessary code and strange error reporting with what's in place, so I think worth revisiting if maybe the compatibility piece was only applicable to older versions.

AFAICT the keyword arguments only ever make their way to the call to `_construct_axes_from_arguments` (which is pretty limited in scope) and enable `copy` to be passed, though that can be done with an explicit keyword argument as well

 > was only applicable to older versions.

@WillAyd : By compatibility, it means compatibility with `numpy`, period.  This is not an issue regarding older versions.  For reference, you can look at the current signature for `transpose` in `1.16`, and you can see what I mean.  That being said, happy to always look at this with fresh eyes and see how we can either improve or streamline.

> Do you know if there was an issue attached to that?

It's been awhile, but if you'd like to dive into the history, you can go in a lot of directions from this PR I wrote then: https://github.com/pandas-dev/pandas/pull/12810 Mostly just trying to simplify existing code. Can always tackle later. 

Sent from my iPhone

> On Apr 2, 2019, at 5:26 AM, Jeff Reback <notifications@github.com> wrote:
> 
> @jreback requested changes on this pull request.
> 
> not sure i understand the purpose of this PR. We can simply remove the use of Panel for transpose rather than actually change what you are doing.
> 
> —
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 I am not sure this does anything except add confusion. would rather just remove Panel methods / whole section that do this.