Write a python or perl program that generates the inverted
index of a set of already preprocessed files. The files are stored in a
directory which is given as an input parameter to the program. Use the files
preprocessed in the previous assignment(s) as test data. Use raw term
frequency (tf) in the document without normalizing it. Think about saving the generated index, including the document frequency (df), in a file so that you can retrieve it later.