For discussion: Future `ellps` defaults?

[busstoptaktik]

In a comment under #4042, @mwtoews says:

Listing ellipsoids could potentially show a similar message.

with regard to freezing the hard coded ellipsoid elements.

But for now, ellps defaults to GRS80, and if I remember correctly (I cannot locate the original discussion - but I think we switched from no default to WGS84 to GRS80, at some time), this is due to "defaulting to whatever the IAG recommends", i.e. currently GRS80.

But IAG's recommendation may change in the future, so we should probably be ready to add future IAG recommendations to the list of hard coded ellipsoids, and perhaps be a bit more clear in the communication that "you should specify GRS80 if you mean GRS80", and only consider leaving out ellps=... as a shortcut for use on the command line and in throw-away scripts.

Thoughts, anyone?