[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
[la.container.eigen] refactor EigenBaseVector
[la.container.common] checked CoW, added DS_FINAL to virtual methods
[la.container.istl] checked CoW, added DS_FINAL to virtual methods
Merge remote-tracking branch 'official/master'

Conflicts:
	dune/stuff/la/container/common.hh
Merge remote-tracking branch 'official/eigenbasevector-refactor'

Conflicts:
	dune/stuff/la/container/eigen.hh
[la.container.eigen] checked CoW, added DS_FINAL to virtual methods
[la.container.istl/eigen/common] fixed non-const get_entry_ref()
[la.container.common] use backend() more often
[la.container.istl] use backend() more often
[la.container.eigen] use backend() more often, optimize mv, fix warning
[la.container.eigen/istl/common] style fixes
[la.container.eigen] changed all IndexType variables back to size_t because warnings where not fixed
[test.la_container] added copy-on-write tests
Merge remote-tracking branch 'tobias/master'