Fixed bug in dmxparse

[dlakaplan]

This fixes #1408, where the R1 and R2 times for dmxparse were the same.

[codecov]

Codecov Report

Base: 62.42% // Head: 62.40% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (56cfbc5) compared to base (d0e2009).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1409      +/-   ##
==========================================
- Coverage   62.42%   62.40%   -0.03%     
==========================================
  Files          89       89              
  Lines       20355    20408      +53     
  Branches     3671     3698      +27     
==========================================
+ Hits        12707    12736      +29     
- Misses       6854     6876      +22     
- Partials      794      796       +2     

Impacted Files	Coverage Δ
src/pint/utils.py	59.24% <100.00%> (ø)
src/pint/gridutils.py	44.21% <0.00%> (-4.75%)	⬇️
src/pint/fitter.py	82.89% <0.00%> (-0.16%)	⬇️
src/pint/models/timing_model.py	82.93% <0.00%> (-0.09%)	⬇️
src/pint/models/model_builder.py	93.50% <0.00%> (ø)
src/pint/toa.py	83.88% <0.00%> (+0.39%)	⬆️

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[dlakaplan]

This is very small, if somebody (@paulray , @scottransom ) wants to check & merge.