Clarification of handling of relative file paths

Describe the feature

We are always using relativ file paths, because absolute file paths are only valid for one specific runtime environment.

The handling of relative file paths shall be clarified. At the moment it is not clear if the reference is the main XOSC file, the current (catalog) XOSC file or another reference point.

This applies to the class "File", which is used in different places: https://www.asam.net/static_downloads/ASAM_OpenSCENARIO_V1.2.0_Model_Documentation/modelDocumentation/content/File.html

It applies also to the property "model3d" in Vehicle, Pedestrian and MiscObject.

Describe the solution you would like

A. Add a statement to the documentation that when using relative file paths that they are always relative to the main XOSC file.

B. Probably add in a later step a property to the class "File" that the reference point can be configured, for example via a parameter from outside.

Describe the backwards compatibility

The suggested solution should be backwards compatible.