adding new function to get a legal value from a warl node (#102)

* adding new function to get a legal value  from a warl node

This function is used to return a legal value for a given set of dependency_vals. If the dependency_vals is empty and self.dependencies        is not empty, then the first legal string is picked and the assignment-string substring is dierctly evaluated to get a legal value.

* Bump version: 3.1.2 → 3.2.0