make sure shape in variables 'zones' and 'retardation' is 3d in Modpath7Sim

[rubencalje]

This pull request fixes a bug where the variable zones (and probably retardation as well, but I did not test this) cannot be supplied to the class Modpath7Sim for a model with a disv-discretisation. Right now this would generate the following error:
ValueError: Util3d: expected 3 dimensions, found shape (nlay, ncpl)

I copied some code from Modpath7Bas to make sure the supplied shape is 3d.

[codecov]

Codecov Report

Merging #1415 (7fe1470) into develop (2ecef16) will decrease coverage by 44.562%.
The diff coverage is 0.000%.

@@              Coverage Diff               @@
##           develop     #1415        +/-   ##
==============================================
- Coverage   74.600%   30.037%   -44.563%     
==============================================
  Files          249       249                
  Lines        53564     52128      -1436     
==============================================
- Hits         39959     15658     -24301     
- Misses       13605     36470     +22865     

Impacted Files	Coverage Δ
flopy/modpath/mp7bas.py	13.333% <0.000%> (-73.901%)	⬇️
flopy/modpath/mp7sim.py	12.400% <0.000%> (-61.781%)	⬇️
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%)	⬇️
flopy/utils/swroutputfile.py	10.714% <0.000%> (-85.079%)	⬇️
flopy/utils/mtlistfile.py	4.620% <0.000%> (-84.700%)	⬇️
... and 206 more