Fix constructor of acset from FinDomFunctor in varacset case

[KevinDCarlson]

The constructor (T)(::FinDomFunctor) where T<:ACSet did not previously add any parts at AttrTypes, which is necessary in case where some of the input's attributes take AttrVar values. We now add such AttrVars, only when they're definitely needed, to avoid dangling AttrVars, which are quite dangerous.

[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

[epatters]

Thanks Kevin!