Allow per-project files to set data path locations (and other environmental variables?)

[ha0ye]

The data path functions check the R environmental variable for the location of where to download and read in datasets that aren't bundled with MATSS.

To add a bit more flexibility, we can also use a per-project config file that also sets these paths, and which will override such global defaults. See weecology/MATSS-LDATS#20 for an example of friction when porting a project to HPC.

[ha0ye]

I think this can be handled by a per-project .Renviron file that overrides the global setting. We should check exactly what variables are overridden (all of them, or just the duplicates?) and document accordingly.