Partial revert "Avoid copying ephemeris data." Fixes #1854

Evan Wardrequested to merge
ephemeris-propagator-performance-1854 into develop

This reverts commit 32f94b9d.

Reverts changes to EphemerisSegmentPropagator that caused the full list of ephemeris points to be copied on every call to interpolate. The original commit was made for #1329 (closed). This commit fixes #1854 (closed).

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