EHN: Left pushforward data migration

[slibkind]

This PR implements the data migration functor ΣF for F: C -> D. ΣF takes a C-Set X and produces a D-Set ΣF(X)

[slibkind]

A few notes:

• This PR implements a struct Functor. Do we want to make this more general and have accompanying methods?
• A potential todo is to refactor the migrate! method to match Σ

[jpfairbanks]

In order to get a code block in the autogenerated docs you can add

    Presentation(CD::Type{T}, AD::Type{S}) where {T <: CatDesc, S <: AttrDesc}

to the first line of the docstring with 4 leading spaces (which is a code block in markdown)

[jpfairbanks]

I just came back to this and realized I've had a pending review for over a week! 😱 I think after my simple fixes are done, this is ready for @epatters to take a look.

[jpfairbanks]

When you create the sigma migration, you could test the dom and codom functions to make sure they get covered by the tests. This will fix the uncovered lines related warnings on this PR.

[epatters]

Thanks Sophie! I just left small comments. I think once you've addressed the outstanding comments this should be ready to merge.

In the longer term, we might want to improve the algorithm for the left pushforward, but I'll create a separate issue for that since it might be a lot of work.

[epatters]

Eventually we'll need to figure out a consistent API design for functors. We already have some other functor types defined in the structured cospan code and there is also this draft PR #220 from Owen. But what is here seems perfectly fine for present purposes.

[epatters]

Thanks @slibkind!