Add Weight and Volume classes to measure module

[David Ross]

Add functionality to manipulate weights and volumes in the same fashion as the Distance class.

[David Ross]

​https://github.com/django/django/pull/10527

[Tim Graham]

I think I accepted the ticket prematurely. Do these classes have anything to do with GIS? If they aren't used by contrib.gis then they probably don't belong.

[David Ross]

Now that you mention it, contrib.gis probably isn't really an appropriate location for these classes. It utilises the base class MeasureBase in the contrib.gis.measure.py file, hence why I placed the classes there. Is there a more appropriate place for these new classes in the code base?

I feel these classes are useful additions for being able to convert units on the fly. For example altering an app users weight units depending on their preferences or region or conversion of volumes for a scientific application.

[Tim Graham]

I don't think it's Django's place to provide these helpers. We have django.utils but this is limited to code that Django itself uses. I'd be surprised if there isn't an existing Python package that provides this functionality. If not, maybe you'd consider creating one.