9 precalculate viability

[nedtaylor]

Change global minimum atom placement method to calculate site viability only on affected sites, rather than each time the placement method is called.

Tested on (M2 MacBook Pro):

• grid = [142, 142, 71]
  atoms to place = 72
  old time > 900 s
  new time = 200 s
• grid = [36, 36, 71]
  atoms to place = 8
  old time = 9 s
  new time = 7 s