specl: CONDITIONAL/IF-THEN (parser and code)

[rburghol]

@timcera

Design:

• Each IF/THEN-able action will contain an enabled data attribute (1/0)
• Conditionals will evaluate, then set the enabled attribute in state for each of it's subordinate action lines
  • This will happen when the conditional gets step() evaluated (so that changes of state are evaluated in the moment)
• @njit code will check enabled state value and return if enabled == 0 (this should be quite fast, despite evaluating many times)