RAP-1132 Add a step to create dynamic spectra

Post DD calibration, you might want to save dynamic spectra (in I, Q, U, and V) in the direction of a particular source of interest. This MR adds a DynSpec step to do just that. This is achieved by averaging over all baselines for each time step and frequency channel.

For each requested source (i.e., direction), dynamic spectra are written to a FITS file, with axes time, frequency, and Stokes parameters (all 4).

Current limitations (to be fixed/added in future MRs):

• No calibration solutions and beam are applied in the direction of the requested directions.
• No polarization selection (similar to wscleanwriter.polarization). Assume that all 4 instrumentatal polarizations are present in the MS and generate spectra for all 4 Stokes parameters.