Added determinant and inverse MX nodes
Redefined gradient(MX,MX) and gradient(SXMatrix,SXMatrix) to use SXMatrix::grad() and MX::grad(). These differ from trans(jacobian(.,.)) in sparsity pattern and when arguments are matrix valued.
Correction to 4eac2a25c80798ae8fcf2d9c793ed32e7aba34d5