Bugfix: limit attack seqs to allocated focus

 - Fix a bug where an attack sequence with a final focus count of -1
   would be legal due to comparing against the wrong variable.
 - Fix a bug where weapon_of_choice couldn't be used more than twice
 - Fix a bug where we could compute boosting permutations for an
   attack in excess of ROF.
Improvement: implement a somewhat better focus structure

 - Dispatch to a focus value based on the type of the attacking
   model. Jacks get 3, Models get 0, Warcasters get their entire focus
   when assessing attack options.
Feature: add support for a 'focus' modifier in the attack params

 - Add a sketchy implementation for the focus modifier, allowing
   attacks to be explicitly evaluated with a user specified ammount of
   focus. The grammar isn't great, if the [+-=] modifiers are used it
   will break but it should work in the simple case of an integer.