Improved caching

[vyasr]

Feature description

Right now all properties are computed every time they are called. We should implement a caching mechanism by which the computed values are stored the first time they are computed.

Proposed solution

We should implement a decorator, e.g. cached_property, that stores computed properties into a global dict cache if they are not found in the cache. We should also implement a second decorator invalidates_cached_properties(property_names) that removes data from the cache whenever a function is run that invalidates some property (e.g. rescaling the area of a polygon invalidates the moment of inertia calculation).