124 add strain information linked with the species csv

[dbirman]

This is a draft PR looking at how to add strain information by adding a StrainModel and Strain class in the species.py file. I also had to add a new filter function to the generator.py file to allow for filtering a dataframe to get back only the unique rows -- this is needed to make the sets of Species unique, even though multiple strains might share a species.

The intention is to use this new feature to separately validate subject.json models to ensure that the species matches the species listed in the strain, when the strain is set.

We can't merge this PR until registry information is added for strains or we remove the registry fields from StrainModel.

[saskiad]

MGI has strain info for mice:
https://www.informatics.jax.org/strain/MGI:3028467
https://www.informatics.jax.org/strain/MGI:2161072

The best way to find them is through this: https://phenome.jax.org/strains
which has a link to the MGI entry for each strain.
(you can also search in MGI and look in the strain list ... if you start with the exact strain name, you'll get it, if you trying something incomplete, you probably won't)

[saskiad]

I think this looks good