WIP: Generalize and export BCValues

[KnutAM]

I think BCValues could be generalized a bit, to both make it easier to follow our internal code, and it could facilitate working with dofs directly for e.g. AffineConstraints (ref #1119).

So far just some initial WIP, but the main idea would be to have an interface for getting the combination of the dof(s) associated with a given coordinate on a boundary entity.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.72%. Comparing base (5f20e7b) to head (808d327).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Dofs/ConstraintHandler.jl	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1152      +/-   ##
==========================================
+ Coverage   93.69%   93.72%   +0.02%     
==========================================
  Files          39       39              
  Lines        6237     6233       -4     
==========================================
- Hits         5844     5842       -2     
+ Misses        393      391       -2     

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