Code generation for LDL and QR

[jaeandersson]

Code generation for LDL and QR is currently not possible due to license restrictions (#2134). To enable code generation, the routines casadi_qr and casadi_ldl, currently residing in casadi_runtime.hpp, need to be reimplemented in a non-copyright infringing manner.

Note that this corresponds to an sparse QR or LDL factorization with all sparsity patterns already given.

[jaeandersson]

LDL should be possible to calculate by conceptually performing the multiplication with row i and column j<=i:

• j==i: d_i = a_ii - sum_{k<i} d_k * l_ik^2
• j<i: l_ij = (a_ij - sum_{k<j} d_k * l_ik * l_kj) / d_j

[jaeandersson]

LDL reimplemented. The new implementation should be more efficient and needs less memory. It also doesn't require the elimination tree to be stored. Instead of calculating L, it calculates the upper triangular L^T, since upper triangular matrices are more natural to store and to operate on.