Resolve RAP-209 "Add diagnostics"

This MR adds various diagnostics to the Rapthor log:

• the measured and theoretical image rms noise
• the image dynamic range (max value / measured noise)
• the image reference frequency
• the restoring beam details

The theoretical noise was checked against the online calculator at https://support.astron.nl/ImageNoiseCalculator/sens.php. It also includes the effect of flagged data. The selfcal convergence criteria were extended to include the dynamic range as well as the noise, as the dynamic range is sensitive to improvements in how well calibrated the brightest sources are (which the mean image noise may not be).