[grid.search] use ReferenceElements instead of deprecated GenericReferenceElements
[la.container.eigen] use BackendType::Index to fix warnings
[la.container.interfaces] fix warning of comparison between signed and unsigned int
[la.solver.eigen] get some values as int instead of size_t to fix warnings
[la.solver.istl] get some values as int instead of size_t to fix warnings
[la.solver.istl_amg] get some values as int instead of size_t to fix warnings