Clean up of detectors

Implemented append, extend, insert methods from super class with a check that makes sure that the interferometers stay consistent.

Adds `set_strain_data_from...` functions to allow easy setting of the
data.

Most of these tests fail because they rely on the old set_data method,
or try to set attributes that aren't settable.

Also fixes a typo

Also adds documentation for buffer_time

- Removes overwrite_psd
- Make the data actually be set

- Add explicit sampling_frequency, duration, and start_time
- Generalise the 1s offset to an argument, buffer_time

In particular, merge bb9b5904 changes to set_data into their new
location.

Adds a set method from an array - this is used by set_data and
inject_signal.

Rather than passing around a list of interferometers, have a defined
set. Motivation: in several places we had dangerous calls to
interferometers[0].attribute which will get confusing if not all IFO's
have the same duration etc. This enforces that they do.

start_time -> epoch

Ensures that a useful error message is reported when the data is not set

- 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