gnu: maths: dune-*: Update to version 2.7

gnu: maths: dune-*: Update to version 2.7

* gnu/packages/maths.scm (dune-*): Update to version 2.7.
(dune-uggrid)[source] Download from Git, as no release tarball for
version 2.7 exists.
(dune-alugrid,dune-subgrid,dune-pdelab)[source] Use commit hashes as no
tags for version 2.7 exist (yet?).
(dune-istl)[source](patches) Add patch required to build version 2.7
without openmpi.
* gnu/packages/patches/dune-istl-2.7-fix-non-mpi-tests.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>