added functions to fetch L and U Factors from IncompleteLUT

Reference issue

Fixes: #2541 (closed)

What does this implement/fix?

It adds the two functions matrixL() and matrixU, which return either the lower or the upper triangular part of the sparse matrix.

Additional information

in the following screenshot you can see an example of how to use the new feature:

I am still in the process of running make check and maby adding unit tests

for documentation, I have an example program I could use for a more extensive documentation, rather than "just" doxygen