Return more information if matrix decomposition fails

Submitted by Christoph Hertzberg @chhtz

Assigned to Nobody

Created attachment 249
Check for non-positve definiteness

Currently SimplicialLLt results in NaN results for non-positive matrices, i.e. it takes the square root of a negative number.

Simple patch which checks for negative values and gracefully exits is attached.

Attachment 249, "Check for non-positve definiteness":
LLtPositiveDefinite.patch

Edited Feb 19, 2021 by Christoph Hertzberg