Allow for returning a dataframe from gget.mutate, w/ more context

[austinv11]

Retained two additional columns for the dataframe: mutation_start and mutation_end. Which avoids the need to manually re-parse mutation strings if needed later on.

Additionally, all the extra info in the dataframe is lost in the python API since there are operations done on the dataframe that are copy-on-write which means that inputted dataframes with the update_df flag set to True only gain two potential columns: mutation_type and wt_sequence_full, whereas I would like all the additional context in my dataframe.

To combat this, I made it so that when out=None and update_df=True, the function will return a dataframe. Otherwise it will revert to the previous behavior.

Let me know if you have any feedback!