Require homomorphism to be unique

[kris-brown]

This PR involves refactoring the backtracking_search interface, which now accepts take, max, and filter keyword arguments.

• take = number of homomorphisms requested (stop the search process early if this number is reached)
• max = throw an error if we find more than this many morphisms (e.g. set max=1 if one expects 0 or 1 morphism)
• filter = only consider morphisms which meet some criteria, expressed as a Julia function of type ACSetTransformation -> Bool

This allows homomorphism to be redefined as:

function homomorphism(X::ACSet, Y::ACSet, alg::BacktrackingSearch; any=false, kw...)
  res = homomorphisms(X, Y, alg; Dict((any ? :take : :max) => 1)..., kw...)
  isempty(res) ? nothing : only(res)
end

The old behavior is recovered via any=true, but this should not be default as it leads to endless user confusion. Thus there is a version bump for this breaking change.

The postprocessing logic for solutions to backtracking search (introduced recently in #918) was simplified and moved into its own function to not clutter backtracking_search.

Note: filter is needed for certain AlgebraicRewriting features, such as trying to get a single (but arbitrary) homomorphism subject to certain arbitrary constraints, such as the dangling condition, NACs, etc.

Also note: there was an unrelated bug in coerce_component for MarkAsDeleted ACSets which was unearthed in this process.

[epatters]

Thanks Kris, looks great! The new behavior should be much more in line with users' expectations.