nodestack: push 'create_cv' functionality responsibility back to ModelChecker

nodestack: push 'create_cv' functionality responsibility back to ModelChecker

It doesn't really make sense to have NodeStack call the 'create_cv' function
for a ModelAction. This commit separates the functionality of 'explore_action'
from 'create_cv', calling each separately from the higher-level
'check_current_action' model-checking function.