Cache calls to units::valid_udunits_prefixes()

[amoeba]

This fixes a performance issue with the entire shiny_attributes app reported in #332.

Before this change, when launching the Shiny app and when editing attributes, a somewhat slow call from the units package, units::valid_udunits_prefixes() would get called. This meant slow startup time for the Shiny app and also clunky editing.

My solution was to cache the slow call on its first call and re-use the cached value. This was implemented with an Environment.

[mbjones]

Thanks @amoeba this looks great.

Looks like R CMD check is failing on R 4 on several platforms, so we should figure that out before we merge this, just to be sure everything is aok. But looks like a quick merge if the GHA tests pass.

[amoeba]

Agreed. @jeanetteclark offered to look into those #332 (comment) and we'll go from there.

[jeanetteclark]

@amoeba I think I found the source of the failing tests, described here: boettiger-lab/taxadb-cache#1

Awaiting clarification from Carl.

[jeanetteclark]

@amoeba and @cboettig I confirmed that the new versions of contentid and taxadb pass locally where they didn't before - but haven't tested on all of the OS's that are failing. Perhaps we specify that the package depends on contentid 0.0.14 and taxadb 0.1.4 then merge. Once those versions are on CRAN presumably our tests will pass 🤞

[amoeba]

Thanks for the sleuthing @jeanetteclark, setting a minimum version on the taxadb dependency here sounds good.

I'd say this PR is good to merge, then.