Verify that input metric has strictly positive eigenvalues

[corentin-prigent]

This update adds a computation of input metric eigenvalues after loading it to verify that they are strictly positive. If not, exit the code.
This is done for mmg3d, mmg2d and mmgs, for both versions of the API functions:
MMG[3D/2D/S]_loadSol()
and
MMG[3D/2D/S]_loadAllSols()

Tests have been made to ensure validity of the checks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.27%. Comparing base (c48f68d) to head (66da499).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #320      +/-   ##
===========================================
- Coverage    50.30%   50.27%   -0.03%     
===========================================
  Files          177      177              
  Lines        47857    47892      +35     
  Branches     10355    10376      +21     
===========================================
+ Hits         24075    24079       +4     
- Misses       16050    16060      +10     
- Partials      7732     7753      +21     

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