Fix mode array

In the pyseobnr waveform generator, the mode array is passed via a mode_array parameter. On the other hand, the gwsignal and bilby interfaces expect the mode array to be named ModeArray. This might have been changed during the gwsignal review, and not propagated back to pyseobnr.

This MR is to allow both names for the mode array in pyseobnr, to also maintain backwards compatibility for people using pyseobnr via its internal interface.

I tested submitting a parallel_bilby run using the mode-array=[(2,2)] option in the configuration file, and printing out the mode array here and it is now passed correctly, I attach the billy logs.

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