REFACTOR runner:
Splitup into ModelRunner (core functionality) and Runner.
Simplifies to provide additional runner classes.
Cleanup model ctors (mutable type => None with ctor init).
Update to new name: use_step_matcher (was: step_matcher).
Update to new name: use_step_matcher (was: step_matcher).
FIX issue #199: behave tag expression bug when or-not logic is used.
Tag expression converted negated tag into an "and" expression.