Return object from test step

[jemrobinson]

Return a ModelTestOutput object from the test_step

Closes #57

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
ice_station_zebra
types.py					67, 81-87, 90-92, 95
ice_station_zebra/callbacks
metric_summary_callback.py					38-44
plotting_callback.py					46-84
ice_station_zebra/models
zebra_model.py					68, 87-90, 109-111, 133-137
Project Total

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[IFenton]

Couple of minor comments, but otherwise LGTM