[LinearSystem] Fix CompositeLinearSystem not invalidating its matrix

[alxbilger]

CompositeLinearSystem never invalidates its matrix, hence it is never inverted. In this PR, the invalidation of the matrix is linked to the invalidation of the matrix used by the linear solver.

Probably broken since 60d44cf indicating a lack of tests.

[with-all-tests]

[ci-depends-on https://github.com/sofa-framework/Regression/pull/119]

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[sofabot]

[ci-depends-on] detected during build #2.

To unlock the merge button, you must

• Merge or close add references for CompositeLinearSystem example scene Regression#119
  For this build, the following CMake flags will be set
  -DREGRESSION_GIT_REPOSITORY='https://github.com/sofa-framework/Regression'
  -DREGRESSION_GIT_TAG='013baf62c3e4a59a87248496317dbd3c37fbe6f3'