Oblig D - Number of Documents

As has been pointed out, the precode for assigment-d has no way of retreiving number of documents for calculating the idf-value. For a quick fix, simply hard-code the number of documents to be 10000, this should make the tests pass.

The tests assume that the tf-idf weighting uses an ntn variant (see slide 41 in lecture 6), so implementations with other variants will most likely not pass the tests, even though they are correct.