Add a base file

[beckyjackson]

Resolves #32

[beckyjackson]

make test passes locally on my machine, so I'm not sure what's going on... I'm passing mro-import.owl as the input to the template and just not merging it, so it should be able to find all import labels...

[beckyjackson]

I'll come back & update this after #34 gets merged in.

[jamesaoverton]

Sorry, I should have looked more closely at this six months ago.

The base file should not be reasoned: "The base file contains all and only native axioms. No further manipulation is performed, in particular no reasoning, redundancy stripping or relaxation." https://github.com/INCATools/ontology-development-kit/blob/master/docs/ReleaseArtefacts.md#release-artefact-1-base-required And I would prefer not to use robot remove anyway.

Instead I want to build the base file just by including everything except external.tsv. It would be interesting to compare that to what you get with the approach you're using in 50081a1