[RAP-314] Use default smoothnessreffrequency of 144 MHz

To make the smoothness constraint interchangeable between datasets a fixed reference frequency is needed. This MR puts the reference frequency at 144 MHz for HBA data if the smoothnessreffrequency is not specified in the parset or set to None, which should be consistent with current behaviour. This way, running with default settings ensures the reference frequency for HBA data is always 144 MHz unless the user specifies otherwise.

LBA behaviour is unchanged and defaults to the old behaviour of using the frequency midpoint for now.