Reversing momentum at frustrated hops

[JanosJiri]

The standard in the field currently appears to reverse the momentum along the NACV when frustrated hop happens. ABIN by default does not alter the momentum, or if revmom=1 it reverses the momentum completely, which is too drastic. Having reversing momentum along NACV during frustrated hops as a default would be good.

Some documentation would be also useful since I had to search for it in the code, which not everyone wants to do. We should have a look at it at some point.

[danielhollas]

Makes sense, thank you.