Add Interpolatable to PositionList

[Stoops-ML]

• Add Interpolatable base class to PositionList
• Add test_positionlist_has_given_epoch

This allows PositionList to have an epoch etc as defined in Interpolatable.

Although PositionList on czml-writer specifically states that it is not interpolatable and does not include an epoch property, the path example on Cesium sandcastle provides an epoch property to the positions input (PositionList type). Seems to me that the documentation is incorrect here.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.22%. Comparing base (6f98435) to head (d212d90).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #120   +/-   ##
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  Coverage   99.22%   99.22%           
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  Files          12       12           
  Lines         774      774           
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  Hits          768      768           
  Misses          6        6           

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[astrojuanlu]

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