Replace usetex_tex with gwdetchar's texify utility

[alurban]

This PR replaces calls to gwsumm.plots.utils.usetex_tex with calls to gwdetchar.plot.texify. Note, this utility will not be available until gwdetchar-1.0.0.

cc @duncanmmacleod

[codecov]

Codecov Report

Merging #272 into master will increase coverage by <.01%.
The diff coverage is 54.29%.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
+ Coverage   39.06%   39.06%   +<.01%     
==========================================
  Files          47       47              
  Lines        7542     7544       +2     
==========================================
+ Hits         2946     2947       +1     
- Misses       4596     4597       +1

Flag	Coverage Δ
#Linux	39.06% <54.29%> (ø)	⬆️
#python35	39.04% <54.29%> (ø)	⬆️
#python36	39.06% <54.29%> (ø)	⬆️
#python37	39.06% <54.29%> (ø)	⬆️

Impacted Files	Coverage Δ
gwsumm/plot/__init__.py	100% <ø> (ø)	⬆️
gwsumm/plot/utils.py	80.77% <100%> (+0.41%)	⬆️
gwsumm/plot/range.py	37.56% <100%> (+0.32%)	⬆️
gwsumm/plot/noisebudget.py	20.83% <100%> (ø)	⬆️
gwsumm/tabs/guardian.py	19.25% <100%> (ø)	⬆️
gwsumm/tabs/management.py	21.74% <100%> (ø)	⬆️
gwsumm/plot/builtin.py	19.17% <25%> (ø)	⬆️
gwsumm/plot/segments.py	15.05% <33.33%> (+0.1%)	⬆️
gwsumm/plot/triggers.py	15.99% <33.33%> (+0.14%)	⬆️
gwsumm/plot/mixins.py	15.15% <50%> (ø)	⬆️
... and 3 more

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