Apply suggestions from code review

Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

Author: cwhanse

tests/test_pvsystem.py

@@ -495,7 +495,7 @@ def test_PVSystem_faiman_celltemp(mocker):
 
 
 def test_PVSystem_faiman_rad_celltemp(mocker):
-    ir_down = 50  # arbitrary value
+    longwave_down = 50  # arbitrary value
     # default values, u0 and u1 being adjusted in same proportion as in
     # https://www.osti.gov/servlets/purl/1884890/ (not suggested, just example)
     u0, u1 = 25.0*0.86, 6.84*0.88