Summary

Made the following changes:

• Further updates to duffing_model_generators and corresponding tests to switch over to using UchannelLO instead of the dictionary version.
• Added additional test in test_duffing_model_generators to call PulseSystemModel.calculate_channel_frequencies - this test will now catch any errors resulting from duffing_model_generators using an invalid format for specifying u channel los.
• Fixed a bug found in writing the above test: in duffing_model_generators, if a coupling strength was 0.0 it would cause an error as it was ultimately being input as None when compiling the PulseSystemModel object, due to incorrect use of python's or function.
• Modified a couple parts of @mtreinish 's PR to directly construct UchannelLO objects (as opposed to using the from_dict method.