Convert output of INIT_CONDS parameter to list of trajectories

[dwhswenson]

This changes the result of the INIT_CONDS.get() method so that it always returns a list of a trajectories. Since this is usually then consumed by scheme.initial_conditions_from_trajectories, this approach will always give a workable result. This is an API break, but since the result on the typical consumer is to allow more input types, I'm not concerned with backward compatibility.

The issue was that sometimes, if calling the INIT_CONDS parameter multiple times (e.g., SampleSet or Trajectory objects from different interface sets/interfaces), this would break the input of scheme.initial_conditions_from_trajectories. While it might be good to also modify that function to allow multiple SampleSets and a mixture of types in the input list, it seemed easier to make that change in the CLI for now. Usually, a Python library-level user has more ability to fix up their input types, whereas a CLI user has no such ability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (de53902) to head (ff15749).
Report is 12 commits behind head on main.

Additional details and impacted files

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##              main       #87   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          110       110           
  Lines         5422      5454   +32     
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+ Hits          5422      5454   +32     

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[dwhswenson]

This is ready for review and comment. Without review, I will merge this after at least 24 hours, no sooner than Mon 26 Aug 15:00 GMT (17:00 my local).

[sroet]

1 question to make sure implicit casting is not suppoted in OPS, feel free to ignore. LGTM