Consider adding gridMET

[mbjoseph]

There are some design decisions to make on this one before we start implementing anything. Here are some options:

1. Add a source argument to cstdata, which can be either "maca" or "gridmet". There are some existing parameters to cstdata that don't make sense with gridmet, like scenario, and models.

2. Refactor cstdata to make dataset-specific cstdata functions, e.g., make two functions:

• get_maca
• get_gridmet

Both of these will return a data frame of files (like we do already for cstdata), that can be passed to cst_df. This interface seems cleaner than option (1) above to me.

What approach would you prefer @WilliamsTravis? Feel free to add suggestions.

[mbjoseph]

Left to do here on the r6_refactor branch which adds support for GridMET:

• [ ] Fix OMPerror (@mbjoseph)
• [ ] Delete top level test dir (@mbjoseph)
• [ ] Fix the documentation issue about things not showing in the \usage section (gives warning) (@mbjoseph)
• [ ] Update the README to describe support for GridMET (@WilliamsTravis)
• [ ] Update the vignette to describe GridMET support and use new syntax (@WilliamsTravis)