Add Snap method to set units to physical quantities

[Daniel Mentiplay]

Is your feature request related to a problem? Please describe

It would be nice to have a method, say snap.physical_units() that sets the snap to give arrays and other values in physical units rather than having to multiply by the appropriate combination of snap.properties['utime'], snap.properties['udist'], snap.properties['umass'].

Describe the solution you'd like

Implement a method on the snap like snap.physical_units() with arguments to set the time, mass, distance units, such that when accessing arrays like snap['position'] they return an array with physical units.

This solution could return a NumPy array in the correct units. Or, perhaps a better option is to return a Pint array which contains the units along with the array.

Describe alternatives you've considered

The alternative is to continue to do this manually with the units provided in snap.properties.

Additional context

We could use unyt or Astropy units instead.