Bugfix: Nonconforming block assembly

[JordiManyer]

This addresses issues similar to #115 when block-assembling.

In addition, fixes multiple bugs related to it:

• Fixes Bug: p4est + :zeromean + block assembly GridapP4est.jl#64
• Fixes Problem running preconditioned stokes in parrallel with periodic boundary conditions GridapSolvers.jl#62

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (86b3d3d) to head (1d9205c).
Report is 30 commits behind head on master.

❗ Current head 1d9205c differs from pull request most recent head 9b61fb7

Please upload reports for the commit 9b61fb7 to get more accurate results.

Files	Patch %	Lines
src/Algebra.jl	0.00%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #147    +/-   ##
=======================================
  Coverage    0.00%   0.00%            
=======================================
  Files          12      10     -2     
  Lines        2529    2633   +104     
=======================================
- Misses       2529    2633   +104     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.