Include options to call FD waveform model with a custom mode array and to call IMRPhenomXHM with multibanding

Added a few lines in _base_lal_cbc_fd_waveform such that if 'modearray' or 'phenomXHMMband' are included in the waveform arguments the waveform model it is called with a custom modearray and if it calls to IMRPhenomXHM then this model is called with a specific value for the threshold of the multibanding.

waveform_arguments = dict(waveform_approximant='IMRPhenomXHM',
                          reference_frequency=50.,
                          minimum_frequency=20.,
                          modearray=[[2,2],[2,-2]],
                          phenomXHMMband=0.) <--- 0 means do not use multibanding 

This would call IMRPhenomXHM using only the 22 and 2-2 modes.

The mode array option can be used for whatever FD model with higher modes.

EDIT: (Greg) added syntax highlighting.