Refactor `init/targets/targets_download`

[kyle-messier]

Refactor 0.4.1 init/targets/targets_download

See for sketch: https://github.com/NIEHS/beethoven/issues/361#issuecomment-2383452703

• [x] initialization
• [x] tar_target per variable. This could be a dataset (e.g. NLCD) or a dataset+variable (e.g. MODIS MOD09GA)
• [x] pattern = configuration

[mitchellmanware]

@kyle-messier For the argument setup, do we want to continue using the loadargs(file_prep_calc_args, ...) method for arguments, where file_prep_calc_args is the path to the calc_spec.qs file? Or should we transition all to a targets-based approach, where all argument functions are stored as targets not file paths?

[kyle-messier]

I think we have one line of code in _targets.R that specifies the time range desired, then that feeds into the first target. Then initialization occurs through targets, so I guess more or less the latter.

[mitchellmanware]

I think we can + should set arglist_download as a target, not as a global object in the _targets.R file. Editing the common time period as a target should invalidate but maintain branches for downstream targets.