Add API documentation on `datalab.objectmodel.ObjectModel`

[PierreRaybaut]

The class datalab.objectmodel.ObjectModel represents a DataLab object model (groups of signals/images).
Each data panel (signal or image panel) has a member objmodel which is an instance of ObjectModel.

ObjectModel has some interesting methods available:

• get_all_objects: Return all objects, in order of appearance in groups
• get_object_or_group: Return object or group with uuid
• get_group: Return group with uuid
• get_group_from_number: Return group from its number
• get_group_from_object: Return group containing object
• get_groups: Return groups
• ...