- Provide a function in the BaseSearchClass to read in the ephemeris
  files
- Some clean up of docstrings

- When Alpha and Delta are not given, use a simple sin
- Also fixes error in DeltsFSpin when Alpha and Delta are given
- Removes Alpha and Delta from MFDv4 call since they are not used

Since each SFT is generated in a for loop, the process can be quite
slow. This allows the user with `pathos` multiprocessing installed to
leverage `-N` cores (set by command line arg) to speed the process up
Setting N=1 or if pathos is not installed, reverts to the usual
behaviour.

- Adds amplitude prefact
- Adds F1
- Reorganise code to aid in future development

Previously, each individual SFT was written separately. This adds
functionality to concatenate all files used `lalapps_splitSFTs`.

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