Assignment 1. Pollutantmean

Write a function named 'pollutantmean' that calculates the mean of a
pollutant (sulfate or nitrate) across a specified list of monitors. The
function 'pollutantmean' takes three arguments: 'directory',
'pollutant', and 'id'. Given a vector monitor ID numbers,
'pollutantmean' reads that monitors' particulate matter data from the
directory specified in the 'directory' argument and returns the mean of
the pollutant across all of the monitors, ignoring any missing values
coded as NA.