Add back in `conversion_function`

I've commented out the conversion_function in bbh_from_gracedb.py as I got this error

Traceback (most recent call last):
  File "/home/gregory.ashton/bilby_pipe/lib_scripts/bbh_from_gracedb.py", line 38, in <module>
    **inputs.sampler_kwargs)
  File "/home/gregory.ashton/bilby/bilby/core/sampler/__init__.py", line 168, in run_sampler
    conversion_function=conversion_function)
  File "/home/gregory.ashton/bilby/bilby/core/result.py", line 572, in samples_to_posterior
    self.posterior = data_frame
  File "/home/gregory.ashton/bilby/bilby/gw/conversion.py", line 651, in generate_all_bbh_parameters
    likelihood=likelihood, priors=priors)
  File "/home/gregory.ashton/bilby/bilby/gw/conversion.py", line 623, in _generate_all_cbc_parameters
    output_sample = generate_spin_parameters(output_sample)
  File "/home/gregory.ashton/bilby/bilby/gw/conversion.py", line 757, in generate_spin_parameters
    output_sample = generate_component_spins(output_sample)
  File "/home/gregory.ashton/bilby/bilby/gw/conversion.py", line 812, in generate_component_spins
    output_sample['reference_frequency'], output_sample['phase'])
  File "/home/gregory.ashton/anaconda3/lib/python3.6/site-packages/numpy/lib/function_base.py", line 2755, in __call__
    return self._vectorize_call(func=func, args=vargs)
  File "/home/gregory.ashton/anaconda3/lib/python3.6/site-packages/numpy/lib/function_base.py", line 2825, in _vectorize_call
    ufunc, otypes = self._get_ufunc_and_otypes(func=func, args=args)
  File "/home/gregory.ashton/anaconda3/lib/python3.6/site-packages/numpy/lib/function_base.py", line 2785, in _get_ufunc_and_otypes
    outputs = func(*inputs)
  File "/home/gregory.ashton/bilby/bilby/gw/conversion.py", line 92, in transform_precessing_spins
    mass_2, reference_frequency, phase)
TypeError: in method 'SimInspiralTransformPrecessingNewInitialConditions', argument 17 of type 'REAL8'

Colm suggests it is that the reference_frequency is an integer rather than float.