Working on CN2_FACT solver...

Added description.
Added Cholesky based linear solver...

... which is gonna be used in the new condensing implementation.

So far, only the in-place Cholesky has been implemented.

+ A small fix in it's base class
Added solve fcn to the Cholesky solver.
Hokay, CN2_FACT... solver code is taking some shape...

... but it does not work. First block row of the condensed Hessian is
correct, but the update of the Q1 block seems to be wrong.
Working on CN2_FACT... solver. Factorization works.