specific stratum type referenced in inversion_length_TS

[rebeccathomas-NOAA]

https://github.com/OSOceanAcoustics/echopop/blob/49a21a8d64415a35d1727a0c789e7ac0694567c5/echopop/inversion/inversion_length_TS.py#L318

Referencing the specific stratum type (stratum_ks) in this function messes things up when one wants to run things with INPFC strata. I'd suggest changing this line to
self.sigma_bs_strata = df_length_counts.groupby(self.model_params["stratify_by"])[

Thanks,
RT
@leewujung

[brandynlucca]

This was addressed in #497, so this can therefore be closed.