indexing error.

[jgallowa07]

Currently, we rely upon the integrity of unique indexing in the data.variants_df for binarymap, but also for Model.get_variants_df(), and Model.add_phenotypes_to_df() Methods. This is not a problem when collapsing barcodes because the index is naturally reset during the aggregation step. However, we have no checks and this caused a great amount of pain when debugging a recent CYP analysis for the Fowler lab. To fix this:

• If we are not aggregating barcodes, we should simply reset the index - rather than a simple copy - to ensure the integrity of indices.
• Make clear in the docs that the index of the passed in functional score dataframe is not maintained in the returned versions of variants dataframe for both Data.variants_df, and Model.get_variants_df method docstring.
• For Model.add_phenotypes_to_df, we need to ensure that the passed dataframe has unique indices - but no need to reset - maintaining the passed index seems like the desired behavior, here. We can do this using a simple check