New module for intrinsic function

Currently, NodeTemplate (toscalib/nodetemplate.py) has a method 'ref_property' to solve node template reference property. Per the latest review,
Thomas:
It could make sense to extract intrinsic function relation into a separate module. Similar to what Heat does in functions.py. Seems to be better for code extensibility and maintainability.

Depends on the functionality of the template method, it should be considered whether to invoke template methods when parsing the tosca yaml file or leaving it for the translator (or orchestrator) to decide.
For example, get_ref_property might reference a runtime property not resolvable on parsing time.
Such a method should be translated to the heat equivalent (this can be relevant for both static and runtime properties).