trying to fix NaN's in zeng

see my comments in the calc.zeng.R file, I added a check for na's in zeta. NaN's begin appearing when surface water and air have exactly the same temperature

@lawinslow @jread-usgs , I have no clue how this function is supposed to work. Is there a reasonable way to prevent C_H from being NaN (line 207)? Does the " & !is.na(zeta)" on line 221 make sense?

Just seems silly that the whole routine would blow up if Air and water temp are equivalent. But I'm not sure if my fixes are actually fixes. Suggestions?