Rename subdomains to subdomain_specs in master/controller to make the
expected type explicit.
Fix the unit tests to include _NTPropagationOnly nodes.
Add the 'test_short' and 'test_med' targets.

test_short: short tests, max 5 sec
test_med: medium tests, max 1 min
Use orientation instead of link tagging when link tagging is turned off by a configuration option.
Make it posssible to set boundary values on ghosts.
Move orientation detection to the subdomain class.
Add first two tests for link tagging with periodic domains.