Allow lal dictionary to be passed through to `_base_lal_cbc_fd_waveform`

If we allow _base_lal_cbc_fd_waveform to be passed a lal.dict rather than creating it itself, it will probably make it easier to include deviation parameters for tests of GR, e.g, #432 (closed).

I think the only change within bilby would be at https://git.ligo.org/lscsoft/bilby/blob/master/bilby/gw/source.py#L324 do

    waveform_dictionary = waveform_kwargs.get("lal_dictionary", lal.CreateDict())

The new syntax would be something like

def non_gr_binary_black_hole(
        frequency_array, mass_1, mass_2, luminosity_distance, a_1, tilt_1,
        phi_12, a_2, tilt_2, phi_jl, theta_jn, phase, non_gr_parameter_1,
        non_gr_parameter_2, **kwargs):
    waveform_kwargs = dict(
        waveform_approximant='IMRPhenomPv2', reference_frequency=50.0,
        minimum_frequency=20.0, maximum_frequency=frequency_array[-1],
        pn_spin_order=-1, pn_tidal_order=-1, pn_phase_order=-1, pn_amplitude_order=0)
    waveform_kwargs.update(kwargs)
    lal_dictionary = lal.CreateDict()
    # insert non-GR parameters
    waveform_kwargs["lal_dictionary"] = lal_dictionary
    return _base_lal_cbc_fd_waveform(
        frequency_array=frequency_array, mass_1=mass_1, mass_2=mass_2,
        luminosity_distance=luminosity_distance, theta_jn=theta_jn, phase=phase,
        a_1=a_1, a_2=a_2, tilt_1=tilt_1, tilt_2=tilt_2, phi_12=phi_12,
        phi_jl=phi_jl, **waveform_kwargs)