extract_u_nk return current state of the file

[xiki-tempula]

I'm working on a workflow for ABFE calculations #111 #114 and is currently working on the preprocessing.subsampling part.
The subsampling method dhdl needs to decorrelate the u_nk according to the column of the current state. However, the data frame returned by alchemlyb.parsing.gmx.extract_u_nk doesn't contain the information with regard to the current state.

I noticed that the alchemlyb.parsing.gmx.extract_u_nk does read state from the file so I wonder if it is possible for the extract_u_nk to return the current state of the dataframe.

I have several thoughts but I want to get the opinion from the community and possible issues with this.

The first is to set the state as metadata of the dataframe but not many people might know this usage. (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.attrs.html)

u_k.attrs['state'] = state

The second is to set this information into the name since it is currently set to 'u_nk', which I think is not that useful

u_k.name = 'u_nk state: {}'.format(state)

The third option is to return the metadata directly, which will break the current API

def extract_u_nk(xvg, T):
    return u_k, {'state': state}

Obviously, one could also recover the state by using the row name
state = u_k.columns.values.tolist().index(u_k.index.values[0][1:])

[orbeckst]

I don't like breaking the API and I am not a big fan of using column names for numerical values (although we might already be doing this somewhere).

The metadata approach seems elegant and does not break anything. If this is a feature of pd.DataFrame then we can use it because we explicitly use DataFrames as our internal data format.