[IFC Tunnel] `IfcVoxelData` alternatives

Voxel Data alternatives:
1. ValueType attribute as a string that has agreed allowed values or enum
2. Define many subtypes, one for each required IFC defined type
3. One generic representation with all measure types possible (IfcValue select)
4. Only external voxel data (ValueType specifies the measure of external)

These were the options discussed and option 1. was chosen as the most appropriate solution. 2. was deemed too rigid if all requirements weren't captured. 3. has the problem of "blowing up" file size and 4. produces problems due to not having a common data structure available for exchange.