fix!(abc): Take multiple DataFrames as inputs

WHY IS THIS CHANGE NEEDED?:

• Function implementations of processes (i.e. process.as_function(...)) are not able to take muliple DataFrames as inputs.
• This affects processes under multisignal.

HOW DOES THE CHANGE SOLVE THE PROBLEM?:

• PostProcessABC.as_function() assumed that there is only one input in the data argument.
• Change treats additional input DataFrames as arguments (*extra_data) after data. This preserves behaviour for core processes that take one DataFrame as an input but also allows multiple DataFrame inputs to be processed.
• Change treats parameters as keyword arguments (**kwargs).
• *args renamed as *extra_data for readability.

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:

• User needs to specify the keyword argument names and can't rely on position alone. Otherwise, an KeyError will be raised.

• I do not add error handling in this case because I think if the error is raised, the solution is obvious enough, i.e.:

  output = multisignal_process.as_function( input1, input2, parameter1)

  will raise an error. Change this to:

  output = multisignal_process.as_function( input1, input2, parameter1=parameter1)

EVIDENCE THAT COMMIT WORKS:

• I ran a script that uses both multisignal and core processes as functions in the format of the example above. No errors returned.
• https://git.ecdf.ed.ac.uk/swain-lab/aliby/postprocessor/-/pipelines/17183 succeeded. This includes unit tests for some core processes.

REFERENCES:

• Issue #28 (closed)