Better way to change the representation of a quantity

[cjrh]

Currently, the way to change the default representation of an object is like so:

>>> from misu import *
>>> x = 100 * kg/hr
>>> print(x)  # Default
100 kg/hr
>>> x.setRepresent(g/s, 'g/s')
>>> print(x)
27.78 g/s

Unfortunately, there seems no easy way to drop the 'g/s' string parameter. One way might be to only have a string parameter (if that can be successfuly evaled.