Select output type of sparsity pattern

[Vaibhavdixit02]

I am not 100% sure this would make sense, thinking out loud here - in some scenarios, for example 10*10 hessian, instead of always returning the sparse form of the pattern having a more structured like diagonal, banded, triangular etc etc matrix would lend itself to nice uses downstream like one could check for such a structure and utilize it in algorithms. Looking at the code it seemed doable since the indices are collected before the sparse matrix is assembled