Feature/required vertices preservation

[Algiane]

The xreg option (regularization of the position of boundary vertices) was not preserving required vertices. The current PR:

1. adds a continuous intgration test that test the preservation of an input vertex (in cmake/testing/code/req-vert-3d.c)
2. adds the check of required vertices in the xreq option.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes are missing coverage. Please review.

Project coverage is 51.26%. Comparing base (96eab63) to head (662302f).
Report is 4 commits behind head on develop.

Files	Patch %	Lines
cmake/testing/code/req-vert-3d.c	0.00%	12 Missing ⚠️
cmake/testing/code/req-vert-s.c	0.00%	12 Missing ⚠️
src/mmg3d/analys_3d.c	0.00%	0 Missing and 2 partials ⚠️
src/mmgs/analys_s.c	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #273      +/-   ##
===========================================
+ Coverage    50.47%   51.26%   +0.78%     
===========================================
  Files          174      176       +2     
  Lines        47158    49124    +1966     
  Branches     10255    11449    +1194     
===========================================
+ Hits         23802    25181    +1379     
- Misses       15633    15934     +301     
- Partials      7723     8009     +286     

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