Arbitrary binning axes

Implementing binning with arbitrary axes, allowing for non-linear binnings.

It works for bin_partition, but still not for bin_dataframe. The reason is I am trying to use nBins as either the number of bins in the old method, but also to pass it arrays directly. This then drops the requirement of binRanges. I will try fix this....

However the numba function works as of now, even if I had to use a workaround for the creation of the target nd-array. I always hit this problem where you cannot create tuples in Numba.

@daviddoji if you can have a look at this would be great! the tutorial notebook has tests which use this new function.