"Create a 3044 x 1 vector containing the mean value, for each gene,
from the Day 0 samples. In other words, this creates a 3044 x 1 vector
from a 3044 x 3 array, with each row containing the average of 3
measurements. Hint: the function mean usually operates column-by-column,
but it can be told to operate row by row. In other words, the commands
mean(X,1) and mean(X,2) will produce different results."