Add additional field line tracing tests

[jameswilburlewis]

We need to add some more challenging tests, including tracing with different models, with actual solar wind parameters, and edge cases like open field lines.

[jameswilburlewis]

• Test all models against IDL results with default parameters
• Test all models against IDL results with actual time-varying solar wind conditions
• Test at least T96 model with autoload=True against IDL results
• Test a scenario with open field lines and verify that behavior is reasonable
• Test that setting the ionosphere boundary radius and maximum trace distance in the ttrace2endpoint call is effective
• Test the diagnostic outputs npts, s_max, reached, nevals, bvec all work as expected
• Test that foot points can be returned if no trace variable is provided
• Test that starting a trace just within the ionosphere radius, going outward, doesn't end prematurely