Add minimal example and associated helper functions

See merge request Monash/tupak!32

- Adds an example showing the simplest number of steps to run PE on
  GW150914
- Adds a helper (rapper) function to quickly get the BBH likelihood
  instance

- Add ability to pass in parameters as either keys or latex labels
- Update truth to not overwrite the given kwarg, but preserve "plot
  injections by default" behaviour.
- Add function to convert a set of parameter_keys into the latex labels
  used by CC

- Set plot=False during tests
- Remove verbose=True from nestle calls

1) Remove the prior from the result object: we already save them as a
text file, so I don't think they need to be in the result. For now, I've
only commented them out with a message as there may be someone tempted
in the future to add it back in.

2) If the saving data does error, it now continues (without writing the
data) and prints a big error message.

Adding ASD/PSD normalization functions to utils

See merge request Monash/tupak!28

Time domain strain implementation

See merge request Monash/tupak!22

# Conflicts:
#	tutorials/create_your_own_source_model.py