Merge branch 'fix/switch-ci-baseimage' into 'master'

See merge request !17

Switch CI baseimage

See merge request !16

(cherry picked from commit 35b1e970)

17e8d53d [doc] dune-multidomaingrid 2.6.0
c6462f38 Switch CI base image for 2.6

Some more small fixes

See merge request !14

Subdomain interface iterators segfault v2

See merge request !13

Iterators now return temporaries, so we have to make sure that the intersection iterators have a
valid cell to operator on.

Revert "Merge branch 'bugfix/subdomain-interface-iterators-segfault' into 'master'"

See merge request !12

This reverts merge request !10

Some small fixes to fix warnings and turn CI green

See merge request !11

That triggers a bunch of internal unsigned / signed comparison warnings, but I think I got them all.

Fix segfault in subdomain interface iterators

See merge request !10

Entity iterators do not necessarily store an entity anymore, so we have to keep a copy around.

Enable Continuous Integration

See merge request !9

Assorted cleanups

See merge request !8

[interface] Adjust MCMGMapper to upstream interface changes

See merge request !7

Remove EntityPointer

See merge request !6

Remove partition iterator type parameter from GridViews

See merge request !5

Update capabilities

See merge request !4

This patches switches to the new way of working with reference elements (get them from the geometry,
value semantics) and switches some grid iterations to range-based for.

I'm not sure whether I always misunderstood this, or whether the method was recently extended to
accept entities of arbitrary codims. Doesn't matter, the gridcheck flagged it, and now the methods
are fixed.

Those were replaced by types() ages ago.

Dune doesn't ship its own conditional anymore, so switch to the std version. While we're at it, also
switch to using the type alias directly and clean up a bunch of other type aliases.

- The layout is now a simple function, not a template parameter anymore
- The layout is now exported by the upstream class, so we don't have to store a copy.

EntityPointer is no more, so purge it.