Optimization in LU::solve: when rows<=cols, no need to compute the L matrix
Remove matrixL() and matrixU() methods: they were tricky, returning a Part,
and matrixL() was useless for non-square LU. Also they were untested. This is
the occasion to simplify the docs (class_LU.cpp) removing the most confusing part.
I think that it's better to let the user do his own cooking with Part's.