added ConservativeSparseSparseProduct.h
overloaded ConservativeSparseSparseProduct to speed up matrix-matrix multiplication.
avoid multiplication with empty matrices.
if one of the matrices involved does not contain nonZeros, we can leave the product
routine.

Conflicts:
	opm/autodiff/ConservativeSparseSparseProduct.h
Merge branch 'PR/performance-avoid-zero-matrix-entries' of ssh://github.com/dr-robertk/opm-autodiff into PR/performance-avoid-zero-matrix-entries

Conflicts:
	opm/autodiff/ConservativeSparseSparseProduct.h