Should `get_save_dir()` return the root experiment directory and/or the specific `vary_across` directory?

[tiffanymtang]

Potential options for the output of get_save_dir():

• Option 1: return the root experiment directory (current implementation)
• Option 2: return the specific vary_across experiment directory
• Option 3: add an argument in get_save_dir() to return either the root or the vary_across directory

See #139 (comment)

[tiffanymtang]

Also related is whether we can make the file paths for varying across experiments more robust. See #139 (comment)