Python: Add functionality to delete object(s) from a collection

[magnesj]

We have support for add object and modification of an object. In some workflows it is required to delete one object or all objects in a container. In the current implementation, the project must be closed and the grid model must be imported again.

• add support to delete a specified object from a container
• Postponed : add support to delete all objects in a container

This will probably require a new message in PdmObject.proto and new functions in RiaGrpcPdmObjectService