Update MultipleSources tutorial with lonlat projection for Rainfields3 (oldname: RadarDemo)

[nikeethr]

Background

The tutorial notebook on multiple sources shows how to merge different data sources. This is from the site_archive_nci only available in NCI currently. It uses the RadarDemo accessor to illustrate how to handle datasets that cannot be merged due to coordinate incompatibility.

Updates in NCI archive

The branch edit-package/archives/nci:radar-accessor-coords (again internal to NCI), where I have implemented projection from x-y to lon-lat from cf-compliant projection attributes derived from the dataset.

There is currently a PR here: https://git.nci.org.au/bom/dset/edit-package/archives/nci/-/merge_requests/15, to be reviewed by @tennlee

The intention is to migrate the projection transform to PET, but this requires some though and refactor to make it mroe generalisable. Until then it may have to remain specific to site_archive_nci.RadarDemo.

Changes in PET tutorial

I've begun (but actually mostly added) changes to the multiple source tutorial - which will also need to be reviewed, (note: it has a hard dependency on the internal radar-accessor-coords branch above which needs to be merged, and/or made available publicly first.)

PR: #96

[nikeethr]

Note this should now be Rainfields3 not RadarDemo. #96 should be rebased and updated accordingly once #106 is merged in

[nikeethr]

Help wanted: will need to go through tutorial with @tennlee (and potentially others) that may be interested in using this data