Renamed internal variables in EF4

  Renamed internal variables in EF4 to avoid the use of single character
variables that might be confused with the single-character rescaled units in
greps.  Also added to comments in MOM_bulkmixedlayer.F90 to clarify the units
of variables that scale with H, using notation like 'in H ~> m or kg m-2'.  Only
comments are changed, and all answers are bitwise identical.