Allow flexibility for multiple Attic/Foundation elements

[shorowit]

A user HPXML had two unvented Attic elements, both with WithinInfiltrationVolume=true. That kind of input is technically sufficient to run EnergyPlus, but our Schematron check disallows multiple WithinInfiltrationVolume elements in case they have different values. Can we improve the Schematron check? If it's not possible to do through Schematron, we can move the error-checking to the ruby measure instead.

Also look at the SkirtPresent and VentilationRate elements, they probably have the same limitation.