Add more information when MatBSR matrix is displayed: size and number of nonzero blocks.

src/faust_linear_operator/CPU/faust_MatBSR.h

src/faust_linear_operator/CPU/faust_MatBSR.hpp

src/faust_linear_operator/CPU/faust_MatDense.h

src/faust_linear_operator/CPU/faust_MatSparse.h

src/faust_linear_operator/CPU/faust_TransformHelperPoly.hpp

src/faust_linear_operator/faust_MatGeneric.h

src/faust_linear_operator/faust_MatGeneric.hpp