mended AUSM+ diff. operator

added solver containing Steger-Warming, Van Leer and Lax Friedrich in one for Euler and Navier-Stokes

Fixes #43

PrefixSum: separate first and second phase for OpenMP implementation and expose performFirstPhase and performSecondPhase methods

Same changes as for the regular Reduction operation...

Fixes #42

Benchmarks can be easily profiled even without this parameter, so it was
just an unnecessary complication.

This is nicer because it more clearly separates data load, computation
and data store. Furthermore, it allows to use instances of std::plus,
std::logical_and, std::logical_or, etc. instead of custom lambda
functions.

It contained only methods for prefixSum and segmentedPrefixSum, which
were identical for Host and Cuda, so they can be easily implemented
directly in Vector and VectorView.

The performance seems to be identical to the code using volatile.

They are not implemented anyway...

This is necessary to be able to run tests in parallel.

The build takes too long because of this and nobody uses it anyway. CUDA
does not support long double in device code at all.