Add Sheaf implementation

[jpfairbanks]

This PR adds a module for sheaves, including abstract types. It has an example for free vector space as a sheaf over Diag(FinSet).

[github-actions]

Review Checklist

Does this PR follow the development guidelines? Following is a partial checklist:

Tests

• New features and bug fixes have unit tests
• New modules have tests that are ultimately called by the test runner (test/runtests.jl)
• Existing tests have not been deleted
• Code coverage >= 90% or reduction justified in PR

Documentation

• All exported functions, types, and constants have docstrings, written in complete sentences
• Citations are given for any constructions, algorithms, or code drawn from external sources

Other

• Style guidelines are followed, including indent width 2
• Changes breaking backwards compatibility have been approved

[jpfairbanks]

@tylerhanks, can you review this? I want to add an example for induced subgraphs next.