Refactor the function process decorator

[sphuber]

Fixes #2171

Note: I have not updated the documentation as that will require more
work in any case and I leave it for the coding week.

The calcfunction and workfunction decorator are essentially
running the same code, with the only difference being the type of
node they use to represent their execution in the provenance graph.
The code has been refactored to a generic process_function decorator
which takes a node class as an argument. The calcfunction and
workfunction decorators are then simply defined by wrapping the
process_function with the correct node class, CalcFunctionNode and
WorkFunctionNode respectively.

The difference in link rules between the two concrete decorators is
also completely defined by those node classes, so the logic of the
FunctionProcess class that is dynamically generated by the process
function decorator can be implemented once. The common functionality of
defining and overriding inputs is tested in test_process_functions.
A process_function should never actually be run, but for testing
purposes we defined a dummy ORM class.

Functionality that is specific to the two concrete decorators, such as
which type of links can be created, is tested individually.

[coveralls]

Coverage decreased (-6.9%) to 62.066% when pulling 1aabfeea92428122a240e396324b563169161081 on sphuber:fix_2171_refactor_workfunction_decorator into 3956299 on aiidateam:provenance_redesign.

[coveralls]

Coverage decreased (-0.02%) to 68.994% when pulling 08cee66 on sphuber:fix_2171_refactor_workfunction_decorator into 3956299 on aiidateam:provenance_redesign.

[muhrin]

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