[la.basic] vectorize induced_norm
[operators.fv] add L2Product
[operators.constructions] cleaner handling of bases in project_operator
[reductors.basic] chose maximum dim_source over all operators for empty RB
[operators] import rb_project_operator
[grids.defaultimpl] small fix
[grids.rect] disallow generation of too small grids

Entities of a grid are not allowed to have multiple subentities
which are identical.
[grids.subgrid] add SubGrid