Suggestion: Add metadata to hdf5 files.

[allgenesconsidered]

I've noticed how some ExcisionFinder functions repeat arguments, such as locus information, Cas enzymes, guide lengths, etc. I wonder if it would be worth it to try and save that metadata to the annotation hdf5 file, so users won't need to retype arguments between scripts.

We could also allow users to override previous metadata if for example they ran annot_variants on several cas enzymes but only wanted to output guides for a select few. Also, adding metadata would allow users to better recall the parameter ran to produce their data in case they need to come back to it at a later time.

One way to implement this would be here:
https://stackoverflow.com/questions/29129095/save-additional-attributes-in-pandas-dataframe/29130146#29130146

[keoughkath]

Addressed with latest pull merge.