Fix incomplete cholesky.

We can't always insert an element on the diagonal if zero, since it might be an explicit zero, in which case the element already exists. Instead, added a method to SparseMatrix: findOrInsertCoeff, that sets an output parameter to indicate if the entry did not previously exist.

Also allow returning the shift parameter from IncompleteCholesky, so we can verify the decomposition.

Fixes #2791 (closed).