Add examples of frequency setup for single dish case

[alle-ira]

We have removed from Sect 2.1 all but the first sentence of the following paragraph. It would be useful to add an Appendix describing a real life example and allow to discuss the various possible implementation strategies.

Complex frequency setups are possible in the same observation, as already mentioned in Sect. \ref{sec:specificities}. For instance, new multi-band receivers acquire simultaneously different frequency bands with different setups (bandwidth, max/min frequency, etc.). Moreover, modern digital backends allow the simultaneous acquisition of more than one spectral window (each with its spectral resolution) inside each receiver band. Such a complex observation should be properly represented in an ObsCore Table.
As an example: the new tri-band K-Q-W receiver being installed on board of the INAF radio telescopes observes simultaneously the K, Q and W frequency bands in the same observations. It can be coupled with a digital backend delivering {\it n} different spectral windows for each of the three observed bands. In this example, if the highest-granularity approach to ObsCore is applied, such a case would be described with $3n$ entries in a ``flat'' ObsCore Table. In case of broadband spectro-polarimetric observations, like the simultaneous study of many emission lines, $n$ may become large. Thus, such numerous ObsCore entries associated to the same dataset might make it difficult for a user to understand the observing setup. The possibility to adopt different levels of granularity in an ObsCore Table might be considered.