New command: Load trajectory

[dwhswenson]

This will give a convenience for loading an existing trajectory from an external file into an OPS file.

Although it is loading into the OpenMM snapshot format, this is mainly because that is internally stored. We should be able to write out to most MD-based engines based on that internal format.

TODO:

• Tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4a9d497) to head (9772eef).
Report is 22 commits behind head on main.

Additional details and impacted files

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##              main       #81   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          110       112    +2     
  Lines         5422      5502   +80     
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+ Hits          5422      5502   +80     

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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 05:00 GMT (07:00 my local).

[sroet]

LGTM!