In eval methods, cache evaluation results and check the cache before evaluating

[jmilloy]

Description
Each eval method should return cached output without further evaluation if possible and should cache the output after evaluation. The coordinates hash for this should be agnostic to the dims order.

Solution
A decorator for eval methods.

[jmilloy]

The decorator will also factor out a few other things from the eval method.

[jmilloy]

See also #140