Assignment 1. Complete function

Write a function that reads a directory full of files and reports the
number of completely observed cases in each data file. The function
should return a data frame where the first column is the name of the
file and the second column is the number of complete cases.
Assignment 1. Corr function

Write a function that 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. The function
should return a vector of correlations for the monitors that meet the
threshold requirement. If no monitors meet the threshold requirement,
then the function should return a numeric vector of length 0.