use space boundaries for area calculation

[DaJansenGit]

We have two types of areas:

• bound area: the area of an element that is bound to IfcSpace
• element area: the area of the element itself
  Those might not be the same. E.g. for overhanging roofs, the element area might be bigger than the bound area, because the overhang part has no space boundaries.

Currently in our implementation, the attributes net_area and gross_area for BPSProducts and ThermalZone instances are calculated by default_ps or values found by finder if available. Those will deliver the element area (see above). But when working with simulation in most cases, the bound area is more relevant. Anyway the element area is still interesting, e.g. to check for missing space boundaries.

Currently, there is no clear separation in our code between bound area and element area. This has to be discussed.