WIP: parameter sets

Modularisation of main() in fit_von_mises.py to small(er) functions or modules.

The main function to use is now analyze() - in the end it should work something like this:
- feed in the data and fitting parameters
- get the fitting results, convergence criteria etc.
- optionally, plot the data and/or the results

It can be used from the command line via fit_von_mises.py or interactively.