ENH: Provide a default key name in read_hdf and to_hdf
For convenience, provide a default name ("data") to quickly store and retrieve data from HDF5.

Now for csv:

``` python
df3 = pd.DataFrame(np.random.randn(10, 5), columns=['a', 'b', 'c', 'd', 'e'])
df3.to_csv("df3.csv")
print df3.from_csv("df3.csv")
```

for HDF5:

``` python
df3 = pd.DataFrame(np.random.randn(10, 5), columns=['a', 'b', 'c', 'd', 'e'])
df3.to_hdf("df3.h5", "data")
print pd.read_hdf("df3.h5", "data")
```

I would suggest to provide a default key name that would make it more convenient to read and write to HDF5 when it contains only one dataset.

In this PR:

``` python
df3 = pd.DataFrame(np.random.randn(10, 5), columns=['a', 'b', 'c', 'd', 'e'])
df3.to_hdf("df3.h5")
print pd.read_hdf("df3.h5")
```

Instead I would not provide a default key in `pd.read_hdf` because it would break backward compatibility.

this is too specific of a change. You might want to do this in your local installation (via monkey-patching these 2 functions), but as a general rule, this can introduce unwanted behavior.

thanks for the idea (I actually do this locally as well too)
 Thanks for your feedback,
I think that the most common and natural user case is to store 1 dataset for each file, so having to specify a key seems redundant and annoying. More experienced users that want to store more datasets in the same HDF5 file can do that easily anyway.
 if you're just tinkering around with an h5 file then you're probably right about the use case.

but if you have huge frames that are related in some way (this basically describes all of the data of the entire field of neurophysiology and i imagine many other sciences as well) i think storing multiple data sets per file is more common. personally, i tend to store "related" data frames (e.g., a collection of experiments related by something like the age of an animal or the experimental condition or something like that) in a single file and then i have CSV file that i read into a frame that i use to basically index into the collection of h5 files, a sort of poor man's scientific database. 

i would rather be forced to provide a key and get an error when i didn't than accidentally overwrite a bunch of data because i forgot to put a key.

I'm not sure if i'm misinterpreting what you wrote above but as for `to/read_csv` you just shouldn't have more than a single frame per file because there's really no general way to tell the two frames apart without just introducing a new serialization format that is _not_ CSV, and so it's difficult to say "`to_csv` doesn't require a key so `to_hdf` shouldn't" because `to_csv` is a single data set format.
 Ok, I would still prefer to have a default key, and get an error if I try to overwrite stuff if I don't set mode to "w", but I see your point, I'll close this issue.
