Fix multiprocessing for Process, Feature, Segment

[hagenw]

Closes #165

Ensure, that the multiprocessing argument of audinterface.Process, audinterface.Feature, audinterface.Segment does not result in an pickle related error, if set to True. Multiprocessing relies on pickling all involved objects to communicate between processes.
The error was originally reported in audeering/opensmile-python#104.

I added tests for audinterface.Process, audinterface.Feature, audinterface.Segment, to detect the error.
Most tests included already the multiprocessing argument, but it was always set to False.

The reason for the error consisted of two parts:

1. In Add per execution process_func_args argument #157 we introduced

signature = inspect.signature(process_func)
Process._process_func_signature = signature.parameters

The problem is that Process._process_func_signature cannot be pickled.
To fix this, I changed the code to

signature = inspect.signature(process_func)
Process._process_func_signature = dict(signature.parameters)

2. When process_func=None is selected, we provide default process functions. Before they were defined on the fly inside the object, which also results in objects that cannot be pickled. This is solved here, by defining the functions outside of the actual objects.

/cc @frankenjoe

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.0%. Comparing base (78a10d4) to head (8b27d33).

Additional details and impacted files

Files	Coverage Δ
audinterface/core/feature.py	100.0% <100.0%> (ø)
audinterface/core/process.py	100.0% <100.0%> (ø)
audinterface/core/process_with_context.py	100.0% <100.0%> (ø)
audinterface/core/segment.py	100.0% <100.0%> (ø)

[maxschmitt]

Looks good to me, thanks for the fix.