MAINT: systematically switch out `doctest-remote-data` with `doctest-remote-data-all`

[bsipocz]

doctest-remote-data-all has been added to pytest-doctestplus 1.4, but also more recent releases fixed a couple of more corner cases. So if we are OK with requiring a rather new doctestplus version, we could ditch all doctest-remote-data directives from the docs files in exchange of just one line at the top.

This would mean testing anything in the file would require remote-data access, but in practice I think we are already doing that as only a very small amount of docs examples can run without reaching out to a service.

[keflavich]

I like the idea but let's keep this as low-priority. It's not such a big deal to add lots of directives and it's a bit painful for me, at least, to keep up with the latest versions of packages in testing infrastructure; it's helpful to be compatible with a wide range even at the cost of some cruft.

Future me will come back and look at this comment and scream, "no the cruft is terrible, upgrade fool!", but I don't care about that guy.

[bsipocz]

Haha, this was triggered by the new heasarc PR, it would not have test failures issues, at least not as wide spread issues, if we already used this approach.

[bsipocz]

But I agree, this is not a high priority, just a nice to have cleanup opportunity.