sim.parse_input expects reference data, but cannot find them

[flo-schu]

AssertionError: Pymob is trying to look up the values of ['expression'] in the reference data, but reference_data=None. Provide reference data if necessary and check if the right input key is used (currently: input='y0').

• Thrown by SimulationBase.parse_input: y0 = sim.parse_input("y0", drop_dims=["time"])
• Caused by missing assingment of the reference_data
• Solution: Assign the observations as reference data; y0 = sim.parse_input("y0", reference_data=sim.observations, drop_dims=[]).isel(time=[0])

Originally posted by @mariegrho in #154

This is a simple one. Basically expand the existing error message in pymob