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 95.78947% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.01%. Comparing base (1b7ca8b) to head (286a79b).
⚠️ Report is 571 commits behind head on master.

Files with missing lines	Patch %	Lines
src/FESpaces/FESpacesWithLinearConstraints.jl	95.78%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
+ Coverage   87.96%   88.01%   +0.04%     
==========================================
  Files         214      214              
  Lines       28302    28333      +31     
==========================================
+ Hits        24897    24936      +39     
+ Misses       3405     3397       -8     

Flag	Coverage Δ
drivers	40.56% <0.00%> (-0.05%)	⬇️
extensions	6.21% <0.00%> (-0.01%)	⬇️
tests	87.26% <95.78%> (+0.04%)	⬆️

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