Improvements to GIZMO implementation

Mixture of small improvements:

• Do not call external functions for min and max.
• Do not use double-precision constants.
• Reduce the number of divisions.

I believe this speeds up the Evrad collapse by 15-20%. @jborrow could you confirm this on your accuracy vs. time plot?

@bvandenbroucke I am likely to have made a mistake somewhere. Could you cross-check that it is all fine?