Add ModelFileGenerator and ModelFile for model data creation

[iKenndac]

This PR is a followup to issue #11, and the discussion in PR #12 and PR #14. Since PR #12 was getting quite stale in terms of its code and how dev is progressing, I decided to start fresh.

Continuing from this comment from @tomasf:

Thanks for the sketch - I have no meaningful objection to any of it. In this PR, I've taken that directly and:

• Changed Generator and Product to ModelFileGenerator and ModelFile, but honestly I'm not that strongly opinionated. Maybe ProductGenerator and Product.

• Added suggestedFileName to ModelFile.

• Added some basic tests.

If you're happy with this, I suggest that I add documentation for the new stuff and adjust the documentation for Model slightly to discuss how it's distinct from this before merging.

[tomasf]

This looks great! I don't have strong opinions about naming either, but I think ModelFile better communicates what it's about than Product.

[iKenndac]

OK great! Give me a little bit to add some documentation and we can get this merged.

Thank you for the discussion along the way and the work you put in.

[iKenndac]

@tomasf Alright, here you go. I also added a static build() function for slightly easier one-shot building, and documented everything. Please give the docs a once-over, but I think it's OK.

[tomasf]

Looks great. Thanks for all the work on this, and for sticking with the design discussion as it evolved. I’m very happy with the end result. I really appreciate your patience and the care you put into this. Thank you for helping make Cadova better!

I’ll be making a new release soon that includes this, along with a minor fix for colors.