Fix EXPPotential time interpolation at/near snapshot times

[adrn]

Describe your changes

Evaluating a time-evolving EXPPotential exactly at a stored snapshot time could silently snap to a neighboring snapshot interval, or raise FieldWrapper::interpolator: time t out of bounds at the first/last snapshot. This showed up for me when testing on some EXP coefficient files I had generated for testing.

This fix adds a small tolerance check to the interpolator when picking the relevant knot interval to use.

Checklist

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