Add a test for backup/restore

See merge request !30

cmake cleanup

See merge request !28

This variable was hard-wired to libdunealugrid.a. This causes errors when
building shared libraries. This patch fixes the issue by removing the variable
and adding the "dunealugrid" target to the LINK_LIBRARIES property of all
targets.

adapt to passing reference elements by value in a compatible way

See merge request !32

In dune-gemeotry 2.6, reference elements are passed by value instead of by
reference. This patch replaces those references by suitable decltype statements
to avoid the deprecation warning and stay compatible to the 2.5 release at the
same time.

The Fallback-implementation Dune::array has been removed from dune-common.

[bugfix] don't fail without SIONlib

See merge request !29

The backup-restore example failed if SIONlib is not found, because the
corresponding read/write methods silently do nothing. This causes the grid to
be restored from an empty stream, resulting in an error.

This patch fixes the issue by restoring the grid from the SIONlib file only if
SIONlib has been found. Otherwise, this option of restoring is silently ignored.

[gridfactory] add insertion index for LevelIntersection

See merge request !31

in the regular case the measure is 1