update(intersect): update days calculation to fix dropped precision

[jlarsen-usgs]

• replace timedelta.days with timedelta.total_seconds() / 86400

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (5239914).
⚠️ Report is 158 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/discretization/modeltime.py	75.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2754      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132626    +8491     
===========================================
+ Hits         68947    96227   +27280     
+ Misses       55188    36399   -18789     

Files with missing lines	Coverage Δ
flopy/utils/hfb_util.py	89.3% <ø> (ø)
flopy/discretization/modeltime.py	79.3% <75.0%> (+2.4%)	⬆️

... and 578 files with indirect coverage changes

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