Merge branch 'feature/fix-subgfs-vtk-output' into 'master'

Feature/fix subgfs vtk output

There are two main changes:

a) we switch from unrelated tags to tags derived from a common base class. This allows to overload the general function with a specilized one for vectorgridfunctionspaces

b) some interfaces of the gridfunctionspace were simming in the subgridfunctionspace,

This fixes #23

See merge request !32