Fargo merge

This is initial implementation of FARGO for piernik. I've tested it on *streaming global* and it yields the same
results. Only 16 times faster...

There are few things that we need to be aware of:

1. Currently mean angular velocity is calculated for each fluid separately. I have no idea whether it's a correct
approach. I haven't checked what happens if two fluids differ significantly in their mean velocity.
1. Potential for selfgravity is only shifted (only integer part), it should probably be interpolated better. However
I've verified that current implementations indeed helps during potential recycling
1. Arbitrary division in azimuthal direction is allowed (on uniform grid of course) however current implentation is very
crude and certainly "overuses" boundary communication routines. Improvements are more than welcome.