Consider support for Numba

[polsys]

Try and see how much faster Numba makes the estimation code. I think there is a lot of potential since the algorithm does a lot of tight loops in Python code.

It would be good to make the integration optional for those who do not have Numba installed. This would mean separate CI test legs for compiled and interpreted versions.

[polsys]

By my initial tests, the benchmark run times are reduced to 25% or even 10% (large_sample_mi) of original. However, this only applies to unconditional MI. I will still have to figure out how to support conditional MI; Numba does not like arbitrary-length tuples.

The Numba annotations are a bit invasive, so I'm thinking of making it a hard dependency. All sensible platforms should be able to run Numba after all. This will cause some maintenance burden as Numba is more strictly coupled to Python/NumPy versions, but it should be manageable.

[polsys]

I still had some trouble with type inference for the n-dimensional grid type, and numba issue 1269 must be worked around.

[polsys]

This is now more of a meta-issue for the overall Numba support. I will try to get basic support working in Alpha 2, and then extend it as Numba matures. The integration needs to be strictly optional.

[polsys]

Closing this. See the discussion in #39.