Isolation of METIS and CSPARSE

[alxbilger]

METIS and CSPARSE are used for sparse solvers. However, they can be better integrated.

METIS

• it could be useful to isolate METIS. Currently, it is an extlib in SofaSparseSolver. So, it is necessary to find_package SofaSparseSolver in order to link with METIS.
• The developer has the choice to use its own METIS library if it is in his PATH, or to use the one in extlib. For consistency between users, only one option could be considered.
• METIS library can be updated: a newer version of METIS is available on https://github.com/KarypisLab/METIS

CSPARSE

• it could be useful to isolate CSPARSE. Currently, it is an extlib in SofaSparseSolver. So, it is necessary to find_package SofaSparseSolver in order to link with CSPARSE.
• CPARSE has been modified for our purpose. What are the changes? It prevents to update the lib. Would it be possible to keep the original CSPARSE library and isolate our modifications?

[alxbilger]

@olivier-roussel Your burden was already partly considered here