ENH: explicit test for Freesurfer & MNE environment variables?

I regularly encounter people who run into highly non explicit subprocess.call() errors due to inexistent environment variable and source (FREESURFER_HOME, 'MNE_ROOT`) or missing Freesurfer licence.

Today, the error was due to the fact that Spyder didn’t have the same env variable as the terminal.

In any case, I think it’d be very helpful if we had a little function that checks i) whether the necessary env variables are present and ii) whether they point to a real path, and throw an error otherwise.

[jasmainak]

I remember it took me a while to figure out what ENH stood for 😉

[larsoner]

I think we have better checks now, feel free to reopen if they are still insufficient