read the data contained in the file referenced by the parameter filename

and return a $*$list of tuples$*$ representing the data

that will look like this: $[(\text{'}$DD$\text{'},1085),(\text{'}$DIS$\text{'},1213),(\text{'}$GE$\text{'},1781),(\text{'}$GS$\text{'},1913),..]$

This is the list referenced as portfolio in the other functions below