re-organize PowerSpace

This branch aims at making the current PowerSpace design simpler.

The following changes have been made so far:

• the PowerSpace members are always fully computed upon construction of the PowerSpace; caching has been removed. This still has performance drawbacks which I hope to address in the future.
• the parameters logarithmic and nbin can still be supplied to the constructor, but they are no longer part of the PowerSpace's state. They are used to compute the binbounds, and forgotten afterwards. (In the current implementation it is possible to set these attributes to values which contradict the actual binbounds, which is inconsistent.)
• pundex is currently not supported, but can be reintroduced if needed.
• "natural" binning (which happens if neither logarithmic, nbin or binbounds are specified to the constructor) has been improved to avoid spurious extra bins due to numerical noise.
• index computation has been accelerated significantly
• the config property has been removed; binbounds is now a direct property of the class.

I'm sure that a lot will have to change before this can be merged; please consider this merge request mainly as a starting point for discussion!

As a quick demonstration of the advantages of the new implementation, run the following commands in both "master" and "index_games" branch and watch CPU time and memory consumption:

a=RGSpace((10000,10000),harmonic=True)
p=PowerSpace(a,binbounds=sqrt(2.)*np.arange(10000.))