Fixed a serious bug. (At least sum was not O.K. when working with an
inout array. We really have to allocate an out array and copy the
results.)

Made mpicollectivecommunication more generic and reduced code duplication.

There are to new functions (allreduce) than can be given a binary
function (e. g. std::plus) representing the calculation.

[[Imported from SVN: r4778]]