link Bacdive isolation source categories to each other to represent hierarchy

[realmarcin]

"Cat1": "#Environmental", "Cat2": "#Aquatic", "Cat3": "#Mangrove

NCBITaxon|Strain -> location_of -> Cat3/Cat2/Cat1 -> subclass_of -> Cat2 -> Cat1

[bsantan]

Edges added in PR #189, however need to consider cases where there is some branching in the hierarchy, e.g. {‘Cat1’: ‘#Environmental’, ‘Cat2’: ‘#Aquatic’, ‘Cat3’: ‘#Geologic’}, {‘Cat1’: ‘#Environmental’, ‘Cat2’: ‘#Terrestrial’, ‘Cat3’: ‘#Geologic’}. We might consider concatenating Cat2 and Cat3 for more specificity