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

Fix up logging

Closes #106

See merge request Monash/tupak!69

Add likelihood evaluations to the posterior data frame

Closes #105

See merge request Monash/tupak!68

- Checks if handlers already exist before adding them
- Add option to turn on/off printing of version information (off by
  default)
- Add call to setup_logger by default (with version information turned
  on by default)

Note - this makes empty calls to setup_logger() obsolete. However, if
you want to log to a file, with setup_logger(label='label') is still
useful.

- Only implemented for nestle and dynesty
- Other samples will have NaN

(partial) Fix #105

Docstring update

See merge request Monash/tupak!63

# Conflicts:
#	tupak/gw/conversion.py
#	tupak/gw/detector.py
#	tupak/gw/likelihood.py
#	tupak/gw/utils.py
#	tupak/gw/waveform_generator.py

Reorganise likelihood and add GaussianLikelihood

See merge request Monash/tupak!65

Attempt to clean up back-end setting

See merge request Monash/tupak!67

Clean up output

See merge request Monash/tupak!66

- Removes warning message
- Add another method to change the backend

Adds a line to print the result and indicate where results are saved

Known interferomters update

See merge request Monash/tupak!64

- Moves the hyperPE likelihood to core
- Adds the GaussianLikelihood
- Update docs
- Update examples