Refactored out the 2D histogram code.
sum paircounter now inherits from Histogram2D -- continuing refactoring to make it easier to add new pair counters.
Clean up code so that it is easy to generalize.
Better commenting to pull out the various pieces of the code.
First pass at plane parallel periodic code.
periodic takes the absolute value.
Unittests and bugfixes.
more care required for periodic function. This is a little ugly.
Test tree against reference implementation.
Comment.