Transforms the single pyfstat.py into a python module splitting the
relevant code into separate sub-files in pyfstat. This should result in
improved readability.

1) Update README installation instructions
2) Put all set_up code into seperate named functions
3) Remove underhanded mpl backend setter

Note that the dynamic injection is currently broken - it appears to only
be injecting the signal in one set of data

- Also adds examples to readme
- Fixes useOffset when ndim>1