Erik Lindahl authored Dec 29, 2017 and Erik Lindahl committed Jan 05, 2018

Introduce a wrapper structure around TNG so we detect
and correct for cases when writing multiple frames with
the same step, or non-zero initial steps to TNG files.
This will avoid frames overwriting each other, and make
sure the time per frame is correct.

Fixes #2189.

Change-Id: I3f44a2681138351d89183e3e196269f2b2f258fc