Pp add warning and tests

[pp-mo]

Sorry, this is what #1 was supposed to be
-- i.e. target you branch so the changes appear back on https://github.com/pp-mo/ncdata/pull/145

Meanwhile, I only made https://github.com/pp-mo/ncdata/pull/147 to check that the changes would work, in advance of your merging them.

So I think if you merge this, and then we close https://github.com/pp-mo/ncdata/pull/147 and go back to https://github.com/pp-mo/ncdata/pull/145,
that is going to be better, because then it's still your branch + your PR.

[pp-mo]

Sorry, this is what #1 was supposed to be -- i.e. target your branch (not main), so the changes appear back on pp-mo#145

Meanwhile, I only made pp-mo#147 check it would work, in advance of your adopting it.

So I think if you merge this, and then we close pp-mo#147 and go back to pp-mo#145, that is going to be better, because then it's still your branch + your PR.