fix(mflistfile): parse budget values missing the exponent "E"

[wpbonelli]

MF6 list files can contain values with three-digit exponents written without the "E", a Fortran quirk for E/ES formats without an explicit exponent width. This caused ListBudget parsing to fail. Tolerate this when parsing budget lines. MF6 will be fixed too, but list files from existing versions will still contain these values.

Close #2855

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.7%. Comparing base (556c088) to head (5f3f15e).
⚠️ Report is 235 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/mflistfile.py	78.5%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2858      +/-   ##
===========================================
+ Coverage     55.5%    73.7%   +18.2%     
===========================================
  Files          644      659      +15     
  Lines       124135   132398    +8263     
===========================================
+ Hits         68947    97696   +28749     
+ Misses       55188    34702   -20486     

Files with missing lines	Coverage Δ
flopy/utils/mflistfile.py	70.2% <78.5%> (+0.7%)	⬆️

... and 585 files with indirect coverage changes

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