Snap val for level-set discretization `PMMG_snpval_ls`

[laetitia-m]

WARNING not to be merged for now because:

• Not fully finished and tested yet - need to properly transfer data
• Need to have mmg PR#280 and ParMmg PR#123 merged to be able to test and merged this PR.
• Some parts of PMMG_snapval_ls function are copy-paste of MMG3D_snpval_ls function. Need to see if we can factorise the functions.
• Need to add ci tests

Creation of the snap value function PMMG_snpval_ls in file ls_pmmg.c when in LS mode.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 63.09524% with 31 lines in your changes missing coverage. Please review.

Project coverage is 63.76%. Comparing base (d11d85c) to head (19f0cc4). Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/ls_pmmg.c	62.65%	26 Missing and 5 partials :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #124      +/-   ##
===========================================
- Coverage    63.76%   63.76%   -0.01%     
===========================================
  Files           47       47              
  Lines        19194    19275      +81     
  Branches      3573     3600      +27     
===========================================
+ Hits         12240    12290      +50     
- Misses        6030     6056      +26     
- Partials       924      929       +5     

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[Algiane]

Thanks!