Drug/Therapy/Drug Group class definitions

Combining a few separate but related issues:

Label requirements -- currently optional in the Drug class
- [ ] Are there records that don't have labels? What sources?
- [ ] Should drug labels be required? We've gone back and forth on this a few times but I'm unsure if there was a particular motivation

Therapy class:
- [ ] Initial definitions sketched here: https://github.com/cancervariants/therapy-normalization/issues/2. How to capture terms as Therapies rather than mixing them in with the Drug class? Better to just call everything a Therapy?

Drug Group class:
- [ ] How to define? Simply provide an Array of Drugs?
- [ ] How to relate to MetaKB proposition classes?
- [ ] How to capture? Possible sources?

Start by reviewing the initial Therapy definitions in issue #2 and the open questions in this issue about labels, Therapy, and Drug Group modeling. No implementation files or tests are named, so first determine the intended class relationships and data sources. Done means the class definitions and label requirements have an agreed, documented design.

Written by the indexing model from the issue text.