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

- Convert markdown readme to rst
- Add pip install entry to installation instructions

Adds fixed_arguments to the wfg

See merge request Monash/tupak!61

- Force version
- Print version information
- Simplify source directory calling