Added code to handle constraint violations resulting, in particular,
from violations of variable value range restrictions signaled by the
restrict_covariate_range trigger function.  This addresses issues
related to redmine task #1958.
Refactoring: Moved shared handle_constraint_violations method out of
traits and covariates controllers and into application controller.
Added Rails-level validation of the level attribute to ensure it is in the allowable range for the given variable.
Merge branch 'master' of https://github.com/PecanProject/bety