Support use of shiny app with external score file

[nmdefries]

Add script to launch shiny app with scores from user-supplied RDS file. User can set path to input score file and output dir via CLI args.

Potential improvements:

• [x] targets could also be used to do the data handling.
• [x] This launch script could be combined with the main run.R. We'd have to decide how to unite user input methods. My preference is CLI args and not an interactive interface, but the interactive version makes the options clearer.

[dsweber2]

I don't have that strong of opinions about CLI args vs not, as long as there's a help message if the call fails.

In light of our earlier discussion about maybe having targets that do score comparisons, it'd probably be better to do this via new targets rather than directly in the shiny app, so that we can also do comparisons with the external scores.

[dshemetov]

docopt may be handy for doing a CLI.

[nmdefries]

• Combined external scores selection with existing interactive interface. Want to move to using CL args, but won't include that here.
• Change all ic80 col names to cov_80 to match historical naming scheme. Not super robust, but I don't want to make this too complicated by asking the user to say which external scores col corresponds to coverage, which to WIS, etc.
• Processing large external score files can cause memory blowups (and seemingly unrelated but consistent errors, Last error: invalid daemons: structure(5L, class = "errorValue") The mirai dispatcher is not running. Please call the start() method of the controller (e.g. your_controller$start() before using methods like push(), collect(), and scale(). If you already did, then the dispatcher may have exited too early. ). Run the splitting step in serial, run GC, and don't keep targets in memory once they're created (setting description). This seems to minimize memory usage.
• Set up a system for passing vars set in run.R into the targets env.
• Used an if (LOAD_EXTERNAL_SCORES) block to do external scores processing. tar_skip and tar_cancel could also be used, but for the shiny app to work correctly, external_names needs to be defined (as an empty vector) and not just skipped. So it seemed like we'd need a if (LOAD_EXTERNAL_SCORES) somewhere. I can look more at tar_skip etc if anyone has strong feelings about this.

[nmdefries]

Test failure appears to be because of epipredict changes. This has been fixed in https://github.com/cmu-delphi/exploration-tooling/pull/32, so will wait till that's on main and then update this branch.