Center points updates

[rjleveque]

Fix a bug and improve center_points.py. Use to center gauges or fgmax, fgout points in computational cells.

Also added utility function center_gauges.py to center gauges if the desired locations are already in a setrun.py function.

[mandli]

Would these be better in a gauges.py module or util.py module?

[rjleveque]

@mandli: Good point, perhaps center_points should be in util.py since it can be used also for fgmax or fgout points, and center_gauges in gauges.py.