Flat Phase Space Layout

[szabo137]

This PR adds FlatPhaseSpaceLayout and the respective build methods for momenta.
The respective coordinates are from the $4N$-dimensional unit hypercube.
Flat means here almost uniformly distributed, i.e. for small masses of the involved particles. They have nearly unit phase space weights, which are uniform for massless particles.
The algorithm implemented here is motivated by the famous RAMBO algorithm (see here). If the coordinates are generated uniformly in the unit hypercube, the momentum building of FlatPhaseSpaceLayout is equivalent to RAMBO.

TODO

• finalize the tests
• add documentation
• remove patches
• cleanup