add oseries models accessor

[dbrakenhoff]

• keep a list of model names per oseries
• list is updated when models are added deleted
• accessor has dict-like access
• initialized when instantiating connector (2 ms per model for arctic)
• add test

e.g.

pstore.oseries_models
# Returns:
# {"oseries1" : ["model1", "model2"], "oseries2": ["model3"]}

pstore.oseries_models["oseries1"]
# Returns:
# ["model1", "model2"]