Bump version to 2.3beta2
[buildsystem,bugfix] Correct installation location for dualmortarbasis.

The headers in dune/localfunctions/dualmortarbasis/dual{p,q}1/*.hh are installed into $(includedir)/dune/localfunctions/*dual*/dual{p,q}1, but they should go into .../dualmortarbasis/dual{p,q}1. This patch fixes this.
(cherry picked from commit 4ae1b62c01311aac9cbe11a7de6926844513babc)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>