EdgePropertyList is empty when loading an existing model

[greschd]

When loading a model with existing edge property lists (e.g. SubLaminate with materials), the resulting list is empty.

Root cause is that the _object_list in the EdgePropertyList is initialized in its __init__, and subsequently updated only when it is directly modified. However, the _from_pb_object constructor of the parent first initializes an empty object, and then only updates _pb_object.