#1291 Removed third argument (Sparsity) for mul
The old behavior is available via the syntax mulold(A,B,C)
#1291 Added new optional third argument for mul
mul now optionally calculates mul(X,Y).setSparse(Z.sparsity()) + Z
#1291 Removed mulold
#1284 Refactored MX matrix multiplication simplifications
#1284 Refactored Matrix<> matrix multiplication simplifications