use vcat rather than append! to combine d_discontinuities and tstops

[oscardssmith]

this version works when tstops is a scalar.

Checklist

• Appropriate tests were added
• [x ] Any code changes were done in a way that does not break public API
• [x ] All documentation related to code changes were updated
• [x ] The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• [ x] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

Test?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8b91268) 81.09% compared to head (308daab) 81.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #445   +/-   ##
=======================================
  Coverage   81.09%   81.09%           
=======================================
  Files          11       11           
  Lines        1476     1476           
=======================================
  Hits         1197     1197           
  Misses        279      279           

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