Eliminate inconsistencies between ThermalBoundary and ThermalOpening

Created by: JoachimBenner

In the FeatureType ThermalBoundary, the properties area and staticConstruction (better construction, see #147 (closed)) are optional. On the other hand, in ThermalBoundary the corresponding properties are both mandatory. There is no real reason for this inconsistency. Furthermore, if an explicit geometry of the ThermalOpening is available (surfaceGeometry), the area can be calculated.

Proposal: Define area and openingConstruction (construction, see #147 (closed)) as optional (0..1) properties.