Two new Writer classes to generate artifacts with either Frequency
Modulation alone, or Frequency and Amplitude modulation

CI was failing when calling the new injection_helper_functions module as
anaconda attempted to update numpy (unknown reason)

A search, based on Reinhard's J0537-6910 work, in which the search is
performed over F0 in a sliding window. Able to track changes in the
power and frequency of the signal.

- Adds latex prior table writer (not found to be useful)
- Add special case of 0 to texify_float
- Expose texify_float in core
- Fix confusing argument order in GridSearch

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.