Draft: deprecate dune.grid.cartesianDomain and add modified dune.grid.domain.cartesian

Reason: setting up grids with a cartesianDomain leads to different boundary ids being used. For example:

• yasp provides 1,2,3,4 (left,right,bottom,top) based on the reference cube's edge number (at least in the python bindings for dune-fem)
• alu provides 1 (the default value)

This MR changes the dgf string that is constructed adding boundary ids like yasp uses also for other grids that support them through dgf.

The main change for user code is using

from dune.grid import yasp
from dune.domain import cartesian
view = yasp( cartesian(...) )

instead of the current

from dune.grid import yasp, cartesianDomain
view = yasp( cartesianDomain(...) )

Of course we could import dune.grid.domain.cartesian into the dune.grid namespace so that the changes are really minimal.