Develop

Added cython versions of brute force pair counters.  I noticed while doing this that doing PBC distance calcs is a significant overhead.  PBC distance calcs slow things down by ~ a factor of 5.  Feel free to take a look to see if you can think of a clever way to speed things up.

Anyway, these things can blaze.  This is probably the best way to do things if a TPCF is being calculated out to radius of order the size of the simulation.  We should do some speed tests to confirm this.  Just something to keep in mind.