#672 ThirdParty/Petsc: write a quick wrapper over MatMatMult. It is not that elaborated: arguments have been given by default; in case we truly need matrix multiplication in the code there is room for improvement using the arguments MAT_REUSE_MATRIX and the one describing the expected number of non zero terms. I used it to in fact validate the current interpolator: P1->P2->P1 isn't expected to work here as pressure was lost in the first interpolation and so zeros were put instead in the P2->P1 operation.