consider better handling of pandas dataframes

[eltonlaw]

Side issue of #54

Needs more research. Maybe have a function to straightforwardly check/parse? Parse out non float columns? Return a pandas dataframe?

Would we need to add Pandas as a dependency?

[gkovaig]

If pandas dataframe passed in, consider:

1. Returning pandas dataframe.
2. Reinstate row names and column names of original dataframe passed in.
3. Process/impute only columns that are float or can be cast as float.
4. Ensure dataframe that is returned retains same sequence (of rows and columns).