[la.interfaces] add check_ind_unique() helper method
[la.interfaces] disallow repeated indices in ind of replace,scal,axpy
[la.numpyvectorarray] add missing assertion
[la.listvectorarray] bugfix
[la.listvectorarray] fix for zero length arrays
[la.numpyvectorarray] bugfix
[la.numpyvectorarray] bugfix
[test] fix VectorArray fixtures
[tests] more VectorArray tests