Bug snow mass err

[jmccreight]

• Closes 'PRMSSoilzone' object has no attribute '_adjust_parameters' #249
• Ensures mass balance for snow on CONUS introducing new (rare) coditions for through_rain when rain but not snow on a snowpack is subsequently evaporated off during the time step. some of this work updates PRMS code slightly
• Removed machine epsilons in favor of slightly larger thesholds which are identical in python and fortran
• Went over consistency of thresholds between PRMS and PWS
• Introduces a more rare mass balance error (4 times*locations) in 2 years

[codecov-commenter]

Codecov Report

Merging #248 (8056e39) into develop (5456603) will increase coverage by 0.61%.
The diff coverage is 90.90%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@             Coverage Diff             @@
##           develop     #248      +/-   ##
===========================================
+ Coverage    78.31%   78.93%   +0.61%     
===========================================
  Files           51       51              
  Lines         6706     6694      -12     
===========================================
+ Hits          5252     5284      +32     
+ Misses        1454     1410      -44     

Files	Coverage Δ
pywatershed/atmosphere/prms_atmosphere.py	89.87% <100.00%> (ø)
pywatershed/atmosphere/prms_solar_geometry.py	91.97% <100.00%> (+0.26%)	⬆️
pywatershed/constants.py	98.36% <100.00%> (+0.08%)	⬆️
pywatershed/hydrology/prms_canopy.py	80.62% <100.00%> (-0.21%)	⬇️
pywatershed/hydrology/prms_channel.py	93.30% <ø> (ø)
pywatershed/hydrology/prms_soilzone.py	80.82% <100.00%> (+0.10%)	⬆️
pywatershed/hydrology/prms_runoff.py	82.97% <75.00%> (+0.42%)	⬆️
pywatershed/hydrology/prms_snow.py	88.11% <89.65%> (+6.92%)	⬆️

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!