Provide defaults for FluxVariablesCache and FluxVariablesCacheFiller

Timo Kochrequested to merge
feature/fluxvarcachedefault into master

Fixes #867 (closed)

Adds a property system helper to check whether a type is undefined/unspecialized for a given TypeTag and a helper for getting that type or a provided type in case it's undefined.

Edited by Timo Koch

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