Refactored code to support local and external material sources
This is the first PR in a series to support external modules. External
modules allow materials to be stored in external data sources such as
databases or web services. No new functionality is introduced in this
PR, rather it is a refactoring of code that will allow for changes to
be introduced in future PRs. Minor performance improvements have also
been made in the model and material managers.
The Python API has been enhanced for many data types to allow for
modification within Python.
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