fix(mp7): fix input file writing

[wpbonelli]

Util2d assumes its first argument is a model, but a ParticleGroup was being passed. This caused an error when writing MP7 input files with certain release configurations. Don't pass ParticleGroup to Util2d as it's not needed, and only check self.model in Util2d if it's not None. Likewise in the ArrayFormat class.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.8%. Comparing base (556c088) to head (a7b6f4c).
⚠️ Report is 105 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2679      +/-   ##
===========================================
+ Coverage     55.5%    72.8%   +17.2%     
===========================================
  Files          644      667      +23     
  Lines       124135   130092    +5957     
===========================================
+ Hits         68947    94715   +25768     
+ Misses       55188    35377   -19811     

Files with missing lines	Coverage Δ
flopy/modpath/mp7particlegroup.py	89.2% <100.0%> (+5.0%)	⬆️
flopy/utils/util_array.py	83.1% <100.0%> (-0.3%)	⬇️

... and 559 files with indirect coverage changes

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