Initial work on cleaning up detectors.py

- Add a InterferometerStrainData class
- Use set_data to set the strain_data attribute of interferometer
- Adds properties to interferometer which call down to the strain_data
- Rename data -> frequency_domain_strain (effects likelihood and utils)
- Removes whitened_data method that was not used
- Move data plotting to method of interferometer and remove duplicate
  code