#424 CIF explorer: migrate to new precondition checker framework + improve 'too many initial states' check

• Best to review per commit.
• Changes:
  • Migration to new precondition checker framework.
    • ExplorerPreChecker no longer checks for conditionally supported features, as that was not used anywhere in the codebase.
    • The check for 'not too many initial states' has been improved:
      • Check now checks states of specification, not each discrete variable.
      • Check now takes automata into account as well.
    • Updated/improved tests.
  • ExprNoSpecificExprsCheck can now check for no derivatives.
  • Added SpecNoTooManyPossibleInitialStatesCheck.
  • ExplorerApplication method order has been improved.

Addresses #424 (closed)