pickling, singletons, and comparison

[FabioLuporini]

We have several objects that should be instantiated as singletons (e.g., LEFT, RIGHT, ... FORWARD, BACKWARD, ...) but they are not. This causes a subtle issue upon unpickling when these objects are compared. For example:

• upon unpickling, an IterationDirection is reconstructed along with a completely new Backward ...
• ... but this Backward will be different from the Backward created in space.py

this causes is comparisons to fail. Which led to PR #813 .

With singletons we should be able to elegantly fix this

[FabioLuporini]

we might be able to turn Tag into a singleton generator and then just have something along the lines of

LEFT = Tag('Left')(...)

[FabioLuporini]

I think what we actually need to do is using python's Enum

[FabioLuporini]

I've promoted this to a bug.

But I'm closing PR #813 for now because I prefer to introduce the concept of singletons so that is comparison works even after unpickling