Preserve ventilation control method

When combining spaces assigned to the same zone, attempt to preserve the
ventilation control method.  If the combined spaces use different
control methods (ie. some spaces "Maximum", others "Sum"), the resulting combined space will use
Maximum.
SDD: Support ventilation per person and floor area
Improve ThermalZone::combineSpaces for ventilation

This is adjustment in response to feedback.  If ventilation method is
mixed for the spaces, then use old method of calculating a
fixed maximum for spaces that use max method.
Fix error with mixed ventilation methods
Merge pull request #579 from NREL/58981912-SDD_DCV

58981912 sdd dcv