fix(ModflowUzf1): update load for iuzfopt = -1

[jlarsen-usgs]

• updates for plot_pathline()
• update test_034, test_uzf_negative_iuzfopt

Closes #1406

[codecov]

Codecov Report

Merging #1416 (5f1e9a1) into develop (1cd4ba0) will decrease coverage by 44.822%.
The diff coverage is 0.000%.

@@              Coverage Diff               @@
##           develop     #1416        +/-   ##
==============================================
- Coverage   74.855%   30.032%   -44.823%     
==============================================
  Files          249       249                
  Lines        53566     52136      -1430     
==============================================
- Hits         40097     15658     -24439     
- Misses       13469     36478     +23009     

Impacted Files	Coverage Δ
flopy/modflow/mfuzf1.py	7.012% <0.000%> (-81.559%)	⬇️
flopy/plot/crosssection.py	4.687% <0.000%> (-59.991%)	⬇️
flopy/plot/map.py	7.563% <0.000%> (-76.381%)	⬇️
flopy/plot/plotutil.py	5.942% <0.000%> (-40.570%)	⬇️
flopy/mf6/utils/testutils.py	0.000% <0.000%> (-93.334%)	⬇️
flopy/mfusg/mfusgsms.py	6.190% <0.000%> (-91.474%)	⬇️
flopy/mf6/utils/postprocessing.py	9.259% <0.000%> (-88.889%)	⬇️
flopy/modflow/mfevt.py	7.812% <0.000%> (-87.007%)	⬇️
flopy/mf6/utils/lakpak_utils.py	2.097% <0.000%> (-85.315%)	⬇️
flopy/modflow/mfhyd.py	13.223% <0.000%> (-85.124%)	⬇️
... and 208 more