If nsegs > 1, a semi-coherent search is performed.

- Adds a simple interface to get grid search given uniform prior
- Fixes detector -> detectors bug (previous versions, the constrains
  where not actually set)
- Adds title functionality to plot2D

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.