Adding Compare design outline

Jusst jotted down thoughts about implementing compare in v3.1
and added to the documentation folder. Will try and get some
mockups sorted too in the near future.
Move uml docs to the design folder
Date Format on Athlete Bests

.. didn't follow the same standard as the rest of
   GC, minor cosmetic fixup.
Fencepost error on CP intervals

.. caused the meanmax curve for an interval to be
   recomputed every time even though we'd gone to all
   those lengths to cache them.