make lmax_nyquist=1 by default for gw_signal

[ta-clarke]

For eccentric analyses using the SEOBNRv5EHM and similar waveforms, there is an argument lmax_nyquist that controls how many modes the waveform should check that the nyquist frequency is obeyed to avoid aliasing around the merger/ringdown. Setting lmax_nyquist=1 omits this check.

The current default is to set lmax_nyquist to the highest \ell mode requested in mode-array but in practice this means sampling will almost always fail for some part of the parameter space.

We have discussed this amongst the eccentricity PE task force and recommend that lmax_nyquist=1 by default in gw_signal.