Refactored GroupBy ASVs
- [X] closes #19733
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I ended up mixing in some cleanup as part of this change
Hello @WillAyd! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on March 08, 2018 at 00:48 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=h1) Report
> Merging [#20043](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/63ce781c9ebfb7d308adb3be62755e2bdf6c0d8e?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20043/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20043      +/-   ##
==========================================
- Coverage   91.71%   91.69%   -0.03%     
==========================================
  Files         150      150              
  Lines       49112    49112              
==========================================
- Hits        45045    45033      -12     
- Misses       4067     4079      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.07% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20043/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=footer). Last update [63ce781...23f0777](https://codecov.io/gh/pandas-dev/pandas/pull/20043?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Travis failures is from `TestTSPlot` and isn't related to this change thanks!