BL Toolkit and versioning

[cbizon]

One goal of bl_lookup is that it will be able to serve older versions of the model. So if you are not ready for the latest release, you can ping bl_lookup with a previous release tag.

The problem here is that we are using the biolink toolkit to access the data. And it has versioning dependencies. i.e. the most recent version of the toolkit cannot handle version 1.2.1 of the model.

So there are a few possibilities:

1. Make biolink toolkit backward compatible to older versions of the model
2. Keep multiple versions of the toolkit around to handle different model versions. Might be somewhat of a pain in terms of managing the python repos? Also will need some docs on which versions of the toolkit handle which versions of the model.
3. Abandon the idea of serving old versions, or modify it to only those versions compatible with the latest toolkit
4. Don't use the toolkit, but I think that then we just end up reproducing this problem in bl-lookup code.

@deepakunni3 @PhillipsOwen thoughts?