Unstructured grid refinement

[JordiManyer]

This is the follow-up PR to #148 . Requires Gridap#adaptivity to be merged first.

Main feature:

We aim to add refinement of generic unstructured distributed models, in the case where refinement glues produced by local (serial) refinement are already consistent. Notably, this happens for uniform refinement (i.e same RefinementRule for every cell in the old mesh).

This PR does NOT intend to

• create an algorithm to make inconsistent glues consistent.
• implement redistribution of unstructured meshes.

Other minor features:

• Added propagation of labels when refining and redistributing. This was missing from Cartesian uniform refinement #148.
• As a spinoff of UnstructuredDiscreteModel refinement, we are now able to simplexify in parallel.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 74 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (a37ada1) to head (aaa971a).
Report is 4 commits behind head on master.

Files	Patch %	Lines
src/Adaptivity.jl	0.00%	74 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #149   +/-   ##
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  Coverage    0.00%   0.00%           
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  Files          10      10           
  Lines        2777    2855   +78     
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- Misses       2777    2855   +78     

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