Add new AirLoopHVAC::addBranchForZone overload

Add new AirLoopHVAC::addBranchForZone(ThermalZone & thermalZone,
StraightComponent & airTerminal) overload.

This fixes an issue with the Ruby and Python bindings where you can not
pass an Air Terminal to addBranchForZone() without first casting it to
optional<StraightComponent>.
Merge pull request #635 from ORNL-BTRIC/new_addBranchForZone_overload

Add new AirLoopHVAC::addBranchForZone overload