Add ability to export single sample calculations to an excel file

[kaylai]

Right now, only batch calculations can be saved out to an excel file. Since the calculate objects contain sample, model_name, and result information, we could allow the user to also export these. This would require the user saved the calculate object to a variable (without .result appended).

Possible solution

We could add a .save (call it what you wish) attribute to calculate classes, which would be a pandas dataframe that contains all of the metadata for that calculation. I think that's all we would need to do, since save_excel looks at the calculations assuming they are pandas data frames and simply passes .to_excel to each.