Method to_df() to transform anndata in pandas DF

[fbrundu]

It should implement the transformation only to a shallow dataframe (X, with obs_names and var_names as indexes).
It only bugs me that the dataframe returned has a slightly different precision than the one used to initialize the AnnData, eg:

> import pandas as pd
> import anndata as an
> df = pd.DataFrame(pd.np.random.rand(10,10), index=range(10), columns=range(10,20))
> df2 = an.AnnData(df, obs=df.index, var=df.columns).to_df()
> (df - df2).abs().max().max()
2.943521026921303e-08

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   65.33%   65.33%           
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  Files           9        9           
  Lines         727      727           
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  Hits          475      475           
  Misses        252      252

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[falexwolf]

Sorry for the super-late response. Thank you for this!

The precision is due to the fact, that dtype='float32' in the AnnData constructor. This is enough precision for anything I've seen in data science. If you set it to 'float64' you won't see this difference.

[fbrundu]

Sounds good, thanks!