move EPSILON to class property

[perrygeo]

This PR moves the EPSILON from a module-level global to a class property. Resolves #18

In order to avoid managing cache invalidation, I removed the concept of cached_properties. Most of those calculations take < 1/2 μs so I didn't see any performance reason for taking on that complexity. For example, the determinant property takes 449ns to calculate and 300ns to lookup from the cache.

Also removed the EPSILON2 global as it wasn't being used anywhere.

>>> from affine import Affine
>>> a = Affine(0.000003, 0, 1000, 0, -0.000003, 1000)
>>> a.is_degenerate
True
>>> a.EPSILON
1e-05
>>> a.determinant
-9.000000000000001e-12
>>> b = Affine(0.000003, 0, 1000, 0, -0.000003, 1000)
>>> b.is_degenerate
True
>>> a.EPSILON = a.determinant * 0.1
>>> a.is_degenerate
False
>>> b.is_degenerate  # other instances are unaffected
True

I'm not fully aware of the history of the codebase and removing the global EPSILON might be a breaking change. I'm not sold on this implementation, just a first cut.

[coveralls]

Coverage decreased (-0.3%) to 97.122% when pulling 616f325 on perrygeo:epsilon-local into d7395eb on sgillies:master.

[perrygeo]

Closing in favor of #20