Migrate skeletal implementations that have multiple candidate targets

[khatchad]

Currently, we fail preconditions for source methods that have multiple viable targets (see edu.cuny.citytech.defaultrefactoring.core.refactorings.MigrateSkeletalImplementationToInterfaceRefactoringProcessor.checkCandidateDestinationInterfaces(IMethod, Optional<IProgressMonitor>). Deciding the true target may require:

1. Asking for user intervention (not so desirable).
2. Applying heuristics (but which ones?)
3. Using machine learning (but how?)

[khatchad]

I believe @masuhar and I discussed the third possibility during my visit to Tokyo Tech. Will look up the notes for that.

[khatchad]

We also should decide if this is future work.

[khatchad]

Deciding that may involve seeing how prevalent this problem is in "the wild."