Add checks for `portalr` data path

[ha0ye]

MATSS uses a different environmental variable than portalr. (though they can be set to the same value, since both portalr and MATSS download individual datasets into named subfolders.

Ideally, we want to accommodate:

• people who set different paths for portalr and MATSS
• people who don't use portalr, but will be getting portal data using the MATSS functions

I think we should add a few checks:

• when calling use_default_data_path() from MATSS, check if there is one set for portalr, and give the user instructions for setting it for portalr if it's missing or is different
• add a check to any functions that download data in MATSS or portalr to give a warning that the default path is going to be used if the data path is not set when the download function is called.