Improve frequency chunking

This MR improves the way in which chunking over frequency is done during the DDECal solves (see RAP-745). Instead of determining the bandwidth of the chunks based on the available memory, a single value of the bandwidth (8 MHz) was chosen that allows all solves to fit within the minimum suggested memory of 192 GB. The use of a single value will avoid changes in the bandwidth from machine-to-machine, which is important for reproducibility (since different bandwidths result in slightly different solutions, mostly near each chunk's edge frequencies, due to the frequency smoothing used during solving that is applied to each chunk separately).

A test run shows that the differences between a run with the new 8 MHz bandwidth and a run that used 12 MHz of bandwidth are negligible in the final images.

Additionally, the restriction on direction-dependent solution intervals + BDA has been removed, as their use together is now supported by DDECal (see RAP-677). This functionality was needed for the memory measurements done for this MR (both DD intervals and BDA are now enabled by default).