Custom floating point precision for boundary conditions.

Optionally choose floating point precision for UBB and ExtrapoationOutflow boundary conditions.

Would pystencils!229 (closed) take care of this automatically?

I would say this MR is probably correct. However, I need to check the integration before merging. It is very important that precision detection is done automatically. We also need to think about the integration with waLBerla. I think it should be possible to hide this from the user as done for the rest.

merged

mentioned in commit 847389c4