Refactor of FESpaceWithLinearConstraints

[JordiManyer]

Changes needed to accomodate master dofs from outside the original FESpace, which is needed in distributed.

Since I was already looking at this, I refactored the whole file to implement the different TODOs that were scattered around the file.

[codecov]

Codecov Report

❌ Patch coverage is 97.15302% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.36%. Comparing base (9cf59d7) to head (137e76b).

Files with missing lines	Patch %	Lines
src/FESpaces/FESpacesWithLinearConstraints.jl	97.15%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
+ Coverage   89.31%   89.36%   +0.04%     
==========================================
  Files         212      212              
  Lines       27013    27040      +27     
==========================================
+ Hits        24128    24165      +37     
+ Misses       2885     2875      -10     

Flag	Coverage Δ
drivers	0.00% <0.00%> (ø)
extensions	0.00% <0.00%> (ø)
tests	89.37% <97.15%> (+0.04%)	⬆️

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