print warning when on `gadi` and `vk83` not accessible

[justinh2002]

[lawrenceabird]

Is this necessary? If we check for $ISSM_DIR, do we need to check that vk83 is accessible? If vk83 is not accessible, the associated ISSM executable would not be loaded and therefore the associated $ISSM_DIR would not be found (and a warning should already be printed). Users may want to use pyISSM with an alternate ISSM executable (and therefore have $ISSM_DIR that is unrelated to vk83. I remember seeing some chatter about this check, but I can't recall the details and just wonder if this is a duplicated/unnecessary check?