Waveform plot fails when using ROQ

The waveform plot fails when using an ROQ source model, this is only after bilby!621 (merged). This simplest fix would be to just not make this plot when using the ROQ. I'm not sure about a better actual solution, it would involve passing the non-roq source model in some way.

Traceback (most recent call last):
  File "/home/colm.talbot/.conda/envs/bilby/bin/bilby_pipe_plot", line 11, in <module>
    sys.exit(main())
  File "/home/colm.talbot/.conda/envs/bilby/lib/python3.7/site-packages/bilby_pipe/plot.py", line 72, in main
    interferometers=data_dump.interferometers, n_samples=500, format="pdf"
  File "/home/colm.talbot/.conda/envs/bilby/lib/python3.7/site-packages/bilby/gw/result.py", line 245, in plot_waveform_posterior
    end_time=end_time)
  File "/home/colm.talbot/.conda/envs/bilby/lib/python3.7/site-packages/bilby/gw/result.py", line 424, in plot_interferometer_waveform_posterior
    fd_waveform = interferometer.get_detector_response(wf_pols, params)
  File "/home/colm.talbot/.conda/envs/bilby/lib/python3.7/site-packages/bilby/gw/detector/interferometer.py", line 303, in get_detector_response
    parameters['psi'], mode)
  File "/home/colm.talbot/.conda/envs/bilby/lib/python3.7/site-packages/bilby/gw/detector/interferometer.py", line 280, in antenna_response
    polarization_tensor = gwutils.get_polarization_tensor(ra, dec, time, psi, mode)
  File "/home/colm.talbot/.conda/envs/bilby/lib/python3.7/site-packages/bilby/gw/utils.py", line 147, in get_polarization_tensor
    raise ValueError("{} not a polarization mode!".format(mode))
ValueError: linear not a polarization mode!