Analysis of pollution data.

function named 'pollutantmean' that calculates the mean of a pollutant (sulfate or nitrate) across a specified list of monitors
'complete' reads a directory full of files and reports the number of completely observed cases in each data file
'corr' takes a directory of data files and a threshold for complete cases and calculates the correlation between sulfate and nitrate for monitor locations where the number of completely observed cases (on all variables) is greater than the threshold