Convert the frequency_domain_source model to a string in the CBCResult

See merge request !355

Previously, the function was added to the save data dictionary.
Eventually causing the function to be pickled and all CBCResults to be
dependent on bilby. This converts it to a string so that people at least
know which source model was used.

Resolve "Add suggested acknowledgements to the README"

Closes #253

See merge request !352

Fixes "The "create your own time domain source model" example doesn't "work""

Closes #290

See merge request !349

Resolve "bilby-0.3.6 doesn't declare dependency on astropy"

Closes #297

See merge request !351

to make sure the core dependencies are specified properly

Also forces pandas==0.23 to avoid bug

Polychord

See merge request !264

Improve cosmology use

See merge request !271

# Conflicts:
#   CHANGELOG.md
#   bilby/gw/__init__.py

Converts the read_in_result function to a class method

See merge request !345

Improves the load_data_from_cache_file function

See merge request !348

basic checkpointing and resuming

Closes #282

See merge request !346

Utils upgrade

Closes #153

See merge request !298

# Conflicts:
#   CHANGELOG.md

- Parameterise by the simpler start_time rather than a trigger time
- Uses the lal functions to read in the lines of the cache file
- Promote warning messages to raised errors if the data is not
  succesfully loaded
- Add doctsings