Transition from MSv2 to MSv4

[smcastro]

I find the Overview documentation of the MSv4 very useful, especially the tables summarising the translation from MSv2 to MSv4 and the key changes, which help me understand the MSv4. As a developer coming from the MSv2 to use the MSv4, I think it would be useful to have some documentation showing main workflows for development using the new MSv4. This could be in Jupyter tutorials or simply a section in the Development section of the documentation. For example, as a CASA developer if I want to use an MSv2 in a test and I apply flags to the MS, for the subsequent test I will unflag the MS to apply different flags later. I believe the grouping will allow me to have several versions of flags, although I didn't quite understand how.