Bug 5981: Plot time axis settings lost after saving and reloading 'twice'
Bug Number: 5981
Release notes (y/n): Yes
For Release Nums: 7.1

Original bug title [6-28-2017]:
  Export then Import of a plot loses the x axis time settings.

It was easy to misunderstand this dysfunction because it required TWO CYCLES
of saving and reloading (the last saved) model file.  (I was able to reproduce
this with PlotPages in a model file; this wasn't a problem with only exported
PlotPage output devices).

The first time the PlotPage was saved, we were refraining from saving the
"axisUnitInfo" record for DateTime-unit axes, due to the mistaken notion that
such records aren't needed.  Even though most of its fields aren't used, such
records contain the unit type of the axis -- this is the only way we know --
at the data model level (Sim/PlotInfo) -- that a certain axis has DateTime-
units.

After reloading that saved model, and saving again, we were THEN not writing
out the following three fields, because those are written for ONLY DateTime-
unit axes, and the knowledge that the bottom-X axis was such an axes had been
lost:

  "$plot" timeScaleAutoOnShow {xBottom} 1
  "$plot" timeScaleFormulaStart {xBottom} {Series Data Start}
  "$plot" timeScaleFormulaEnd {xBottom} {Series Data End}

See fix in Sim/PlotInfo.cpp, method: PlotInfo::dump(cwostream&);
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