improve speed of Ax=b solve operations for matrices

perform checks similar to matlab: https://de.mathworks.com/help/matlab/ref/mldivide.html

for dense matrices:

• not square operator -> use QR or SVD
• implement check & solver for permuted triangular matrices
• not Hermitian -> LU solver
• all positive / all negative diagonal -> try cholesky
• default to LDL solver otherwise

for sparse:

• check for diagonality -> scale input
• check bandwidth < threshold -> banded solver
• implement check & solver for permuted triangular matrices
• not Hermitian -> LU solver
• all positive / all negative diagonal -> try cholesky
• default to LDL solver otherwise