Feature/mmg install private headers

[Algiane]

Add possibility to install private headers of Mmg

This PR adds the possibility to install private headers of Mmg by enabling the MMG_INSTALL_PRIVATE_HEADERS CMake flags to ON. It allows to use private Mmg functions in other projects built on top of Mmg.

It is linked to:

• PR #182
• issue https://github.com/MmgTools/ParMmg/issues/31
• PR https://github.com/MmgTools/ParMmg/pull/80

It also clean the header installation of Mmg: now, instead of being copied in multiple locations to be found, common headers of Mmg2d, mmg3d and Mmgs are installed in mmg/common directory of CMAKE_INSTALL_PREFIX directory.

Note that:

• Usage of private functions is done at your own risk: we don't ensure the retrocompatibility of prototypes, the support or the documentation of such functions;
• It is not tested on windows architectures (and probably not supported)

[Algiane]

@prj-

I hope that this PR will satisfy your need and will free you from the charge to maintain your fork of Mmg. I would be very thankful if you can test this branch with the https://github.com/MmgTools/ParMmg/tree/feature/Find-Mmg-private-headers branch of ParMmg.

You will have to clean your previous Mmg install as the branch propose a new tree structure for the Mmg installation and to enabled the MMG_INSTALL_PRIVATE_HEADERS CMake's flag at Mmg configuration.

Thank you by advance of any feedback.

[prj-]

Re-compiling now, thanks for the heads-up.

[prj-]

/media/psf/repositories/petsc/arch-linux-c-debug-real/externalpackages/git.parmmg/src/parmmg.h:50:10: fatal error: libmmg3d.h: No such file or directory
   50 | #include "libmmg3d.h"
      |          ^~~~~~~~~~~~

[prj-]

It looks like you are missing some fixes for ParMmg from https://github.com/prj-/ParMmg/commits/jolivet/fix-compilation-3.19.0.

[Algiane]

/media/psf/repositories/petsc/arch-linux-c-debug-real/externalpackages/git.parmmg/src/parmmg.h:50:10: fatal error: libmmg3d.h: No such file or directory
   50 | #include "libmmg3d.h"
      |          ^~~~~~~~~~~~

In branch feature/Find-Mmg-private-headersof ParMmg, lines https://github.com/MmgTools/ParMmg/blob/8f156cb7409d27d3ed628e7ccb90c70d6d6d4a23/CMakeLists.txt#L293-L294 add the mmg/common and mmg/mmg3d sub-directories of the installation directory of Mmg (that you can specify through the MMG_DIR variable) to the parmmg include directories.

It should be sufficient to not have to add the path the libmmg3d.h file.

In practice I don't care if I have to add the path toward the libmmg3d.h file but it is more complicated for some of private headers as inclusion path may be different in Mmg than in ParMmg.

[Algiane]

It looks like you are missing some fixes for ParMmg from https://github.com/prj-/ParMmg/commits/jolivet/fix-compilation-3.19.0.

Thanks for the reminder. As I have only few days to work on Mmg/ParMmg, I am focusing on fixing important things for the next Mmg release and for now I don't have the time to review your propositions of improvement in the cmake of ParMmg.

Best