The storage of bigunsignedint was changed to std::uint16_t in commit
541b3dfc. This implements the same
change for the MPITraits<bigunsignedint<k>>

Note that this assumes that
  MPITraits<std::uint16_t>
is defined. This is the case as long as uint16_t maps to one of the
standard integer types (unsigned char/short/int/long).

Thanks to Markus for pointing this out.

Reference: https://dune-project.org/flyspray/index.php?do=details&task_id=1657#comment6393

see FS#1055 for details


[[Imported from SVN: r7055]]

[[Imported from SVN: r6925]]

[[Imported from SVN: r6792]]

mpitraits.hh forward declares std::pair, which is illegal by the standard, and clang stumbles over that.
Fixes FS#1121.
(Thanks to Steffen Müting for the patch)

[[Imported from SVN: r6791]]

[[Imported from SVN: r6730]]

according to the standard std::pair is a struct

[[Imported from SVN: r6727]]

[[Imported from SVN: r6269]]

[[Imported from SVN: r6108]]

in dune-istl in revision 1220.  The new file here is intended to be
merged with mpicollectivecommunication.  That will happen in a separate
step.

[[Imported from SVN: r6018]]