Review and document accessor metadata provenance

[timstaley]

cf https://redmine.astron.nl/lofar_issuetracker/issues/6598

For any given image/observation, we pull in important items of metadata like central frequency, bandwidth, time of observation, etc.

However, we currently don't document exactly how we pull this from different data sources (e.g. FITS, CASA image) - you have to dig into the source to find out the details. It would be nice to review this and briefly write it up in the docstring for each class.

[AntoniaR]

As we will only handle fits images in R7, this is only required for that accessor.

[HannoSpreeuw]

@suvayu and me have recently reimplemented the DataAccessor class, while keeping issue transientskp/tkp#53 open since some of its derived classes may require redesign.

Moreover, the accessors folder and the corresponding unit tests will be moved from TraP once this PR transientskp/tkp#619 is merged. accessors will persist in PySE, with the unit tests added.

This means we should be adding the required docstrings.
So we could turn this into a PySE issue, but possibly not worth the hassle.

[AntoniaR]

Great!

It's quick and straightforward to move the issue to PySE if that is useful (I already moved one issue)

[HannoSpreeuw]

O perhaps that'd be neat. Please do so.