Convert {worldmet} to the Global Historical Climatology Network

[jack-davison]

Note that the GHCN still only has majority US data, so this PR is a draft.

This PR fixes #32.

This PR is quite a large overhaul of {worldmet} to use the GHCN over the ISD. This PR:

• Adds import_ghcn_stations() and import_ghcn_codes() to access GHCN metadata.
• Adds import_ghcn_hourly() and import_ghcn_daily() to access data from the GHCNh and GHCNd.
• Deprecates, but does not remove, getMeta(), importNOAA(), and weatherCodes.
• Refreshes the {pkgdown} site, package documentation, vignette, and README files to reflect these changes.

Back Compatibility

The choice to deprecate getMeta() and importNOAA() rather than replace their internals was done for a couple of reasons:

• Leaving importNOAA() where it is ensures back-compatibility for users using {worldmet} in production.
• importNOAA() and getMeta() aren't particularly descriptive names - import what from NOAA? Get what metadata? More evocative function names make for nicer, more readable scripts.
• getMeta() is an awkward function because it isn't clear whether it returns a map, a table, or just prints both, leading to the map and returnMap args. import_ghcn_stations() has the return arg, wherein a user can specify "map", "table", or "sf" (the latter of which returns a spatial dataframe).

Future

There are lots of data sources that are part of the GHCN that aren't accessible through {worldmet}. I think two that could be of interest are:

• Synoptic Summary of the Day (https://www.ncei.noaa.gov/oa/synoptic-summary-of-the-day/v2/doc/ssodv2_DOCUMENTATION.pdf) - much like the GHCNh, it doesn't appear to be finished, but could be nice to provide (a global day summary, to compare the same 24-hour period in - e.g., - the UK and the US, rather than the local 12 AM - 11 PM).

• GHCN Monthly (https://www.ncei.noaa.gov/products/land-based-station/global-historical-climatology-network-monthly) - these are all in great big ZIP files so would have to work differently, but why not complete the set?

Questions

• Currently the functions are in snake_case rather than camelCase like the rest of the {openair} toolkit - in part because importGHCNhHourly() looks dreadful written down. I'd probably prefer camelCase for consistency.

[davidcarslaw]

thanks jack - important step for the future...