Adopt Community Units Package

[mandli]

GeoClaw currently contains its own, simple unit package for dealing with a number of units found in GeoClaw including ways to convert. There are a number of community packages that are well supported that do this same thing with various pros (and cons). One that comes to mind is Pint. I think that the clawpack.geoclaw.units package is serving its purpose but it may be nice to not have to depend on our own maintenance and gain additional capabilities. Implementation would require adding the package to the required packages and converting references to units to whatever package we would decide upon.