First version of an abstract reduce function

This MR introduces an abstract reduce function which could replace e.g. the getMaxValue function.

The std::function might reduce the runtime of the function. I made some benchmarks with the allGather version of this.

Here is the average runtime in seconds per process for a very simple benchmark which performs the getMaxValue 1000 times with 4608 processes (64 nodes * 72 cores). Note: l

	getMaxValue	w.o. MPI		abstract getMaxValue	w.o. MPI
first run	0.20 s	0.12 s		0.63 s	0.17 s
second run	0.17 s	0.12 s		0.61 s	0.17 s

When using allReduce instead of allGather these are numbers for commit 6e12c653. again 1000 times with 4608 processes (64 nodes * 72 cores)

	getMaxValue	w.o. MPI		abstract getMaxValue	w.o. MPI
first run	0.18 s	0.11 s		0.22 s	0.14 s
second run	0.20 s	0.11 s		0.21 s	0.14 s