Fix: Plot step curve symbols were shown at begin and end of timesteps [6033]
Bug Number: 6033 and 5953
Release notes (y/n): Yes
For Release Nums: 7.2, maybe 7.1.6

Gnats 6033: Plot step curve symbols shown at begin and end of timesteps.

This bug is the result of a special "Step" curve style nuance of the Gnats 
5953 fix for RiverWare 7.1.  Gnats 5953 (of seven months ago, Apr 2017) 
addressed the following problem.  Plot curves with a good number of data 
points (e.g. 500+) and a Dotted or Dashed Line Style sometimes appeared as 
solid lines.  See this webpage:

  http://cadswes2.colorado.edu/~philw/2017/Plot/Dense/DenseProb.html

The solution to Gnats 5953 was to apply the Qwt "Weed Fitter" (point filter) 
to slot curves.  However, Qwt fitters cannot be applied to Qwt "step" curves.
We handled that by implementing such curves as "line" curves WITH EXTRA 
POINTS INSERTED to implement the steps.  That's actually a problem if point 
symbols are shown on the curve: Gnats 6033.

To solve THAT problem, we are backing off of the 5953 solution on slot curves
using: (a) Step curve style, AND (b) non-empty point symbols*.  This makes 
sense because showing point symbols on DENSE curves creates the same problem 
that the Qwt Weed Fitter was intended to address, so there's not much point 
in supporting that fix (to patterned lines on dense curves) if point symbols 
are shown.

*While we're at it, we're also skipping that solution for Step style curves 
having Solid or Blank line styles.  (The 5953 solution is technically needed 
only for _patterned_ lines). 

This logic is now encapsulated in this new method:
  void SlotCurve::updateQwtCurveAndSamples().

These changes are initially being committed to a temporary branch, 
Bug6033_71, on top of Release_71.  More testing is needed, also with 
Gnats 5953 test models.

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