[dune-functions] Function space of constant functions

[Simon Praetorius]

Hi everyone,

I have recently implemented a minimal function space for dune-functions 
that represents essentially something like span{1}, so a space of 
constants or of constant functions. I wanted to have this in the context 
of dune-functions functionspacebases to embed these constants in a PDE 
setting. Applications include:

- Neumann problem with integral constraint \int u dx = 0
- Stokes equation with constraint on the pressure (similar to point 1)
- Coupling of scalar equations involving integral terms with a PDE (e.g. 
Cahn-Hilliard SAV scheme)
- ...

See https://gitlab.mn.tu-dresden.de/spraetor/dune-scalarbasis

Maybe someone is interested in this. Probably there are also other ways 
to achieve the same, but this was for me just an experiment how to 
include this in the functionspacebases setting.

If there is wider interest in this feature, I could merge this into the 
dune-function repository.

Best,
Simon