Add "worldclim" and "cmip6" climate caching/access functions

[brownag]

Analogous to how caching/raster access functions for PRISM, ISSR800, Daymet work

• [x] newhall_worldclim_cache()

  • Download and cache worldclim global data in user directory; add {geodata} to Suggests

• [x] newhall_worldclim_rast()

  • Return reference to full cached dataset

• [x] newhall_worldclim_subset()

  • Convenience method for accessing subsets of cached dataset conforming with input spatial object

• [x] newhall_cmip6_cache()

  • Download and cache CMIP6 future climate model data in user directory

• [x] newhall_cmip6_rast()

  • Return reference to full cached dataset

• [x] newhall_cmip6_subset()

  • Convenience method for accessing subsets of cached dataset conforming with input spatial object

[brownag]

I would also like to add CMIP6 (future climate) sources also provided via {geodata} package

[brownag]

Done in https://github.com/ncss-tech/jNSMR/commit/3e772fc047a8c9dfd1d8fc43ccc94fd62512b8dd, will put together a sample dataset (that folks can use without caching) to include in the package after a bit more testing