Change default terminal priority

Instead of setting terminal heating/cooling priority to 1 when added to
zone, assign as the last priority.  Default priorty is thus controlled
by the order equipment is added to zone.  The API still allows changing
priority after the fact.
Merge pull request #972 from NREL/69202022-TerminalPriority

Change default terminal priority

This will alter default behavior and users/developers may need to update their workflows to accommodate this change. However, this must be done since there is no current way to alter terminal priority from the GUI. @kbenne will send out an email to all OpenStudio developers.