BUG fix in solve_cholesky_lower(gram_compact matrix) case. add many unit tests, making sure all gram_compact code is covered by tests.
BUG fix in solve_cholesky_lower(gram_compact matrix) case. add many unit tests, making sure all gram_compact code is covered by tests.

sebastien-bratieres/pygpstruct,sebastien-bratieres/pygpstruct