[la.container.istl] changed get_entry method to return 0 if indices are not in the density pattern
[test.la_container] renamed one variable
[test.la_container] added matrix tests
[test.la_container] reverted splitting into dense and sparse matrizes
[la.container.eigen] fixed unsafe casts, added missing return in dot-function
[la.container.eigen] quick and dirty fix for CRTP error