Seeds : redefined what the 'diagonal' field means.  NOTE that no code has
been updated for new system yet....
1. updated 'diag_neighbor' method for new 'diagonal' system
2. updated calculation of mid-point, store value in seed
Moved code to split a seed into two diagonal halves --> seed.lua
(method of the SEED_CLASS).  Similarly moved code to join two halves
back into a square seed.
Further fixes for new 'diagonal' field -- I think that is everything now.