bump version to 2.7-git [ci skip]

See merge request !79

[ci] stop testing with Debian 8

See merge request !77

Allow full tet ruleset

See merge request !75

'Broadcast' wants a plain pointer, a std::unique_ptr won't do.

UG contains two implementations of green closures of tetrahedral
elements, and the TET_RULESET macro allows to switch between
the two.  As I recently learned from Peter Bastian, we have been
using the inferior implementation all along, and we should be
enabling TET_RULESET in the future.  As a first step, this patch
allows to set TET_RULESET from the CMake flags.

Incidentally, setting TET_RULESET fixes

  core/dune-grid#27

Beware: when TET_RULESET is set, UG wants to read a table from
the file RefRule.data (in uggrid/lib/ugdata).  This file needs
to be in the current directory, otherwise UGGrid will abort.

Remove unused method YAlignment

See merge request !74

More cleanup

See merge request !73

Only three functions implementing commands still in use by dune-grid remain.

More cleanup

See merge request !72

This addresses the following warnings from gcc-7:

    ../parallel/ddd/xfer/cmdmsg.cc:476:7: warning:
    unused variable ‘i’ [-Wunused-variable]

    ../parallel/ddd/xfer/pack.cc:395:48: warning:
    variable ‘recvProc’ set but not used [-Wunused-but-set-variable]

    ../parallel/ddd/xfer/unpack.cc:1226:7: warning:
    unused variable ‘lenSymTab’ [-Wunused-variable]

    ../parallel/ddd/xfer/unpack.cc:1482:9: warning:
    unused variable ‘ret’ [-Wunused-variable]

This way dependencies installed via `duneci-install-module` are built
with the same options as the module to be tested.

More cleanup

See merge request !70