Add data imputation capability

[tjrocha]

Allow users to fill in missing data in the input data arrays. Propose

• using the data imputation methods in the statsmodels package
• and putting this feature in the DataAnalysis GUI for plotting and diagnostics

[jslanini]

I'm cautiously supportive. My concern is introducing bias into the dataset through imputation. Should we examine alternative imputation methods? See https://www.omicsonline.org/open-access/a-comparison-of-six-methods-for-missing-data-imputation-2155-6180-1000224.php?aid=54590

[tjrocha]

Propose having the GUI update to aid in the QA/QC/Acceptance of the imputed dataset. Something like:

• show how many data-points were filled-in
• plot time-series data for datasets with the imputed data -- color imputed data separately
• allow user to overwrite main raw-data dataframe with the imputed dataframe

stretch goal(s)

• integrate other imputation methods and allow user to customize which imputation method is used