Rename random circulant gram to random circulant gram matrix

[vprusso]

In order to maintain consistency with the rest of the functions in this project, we should add the term "matrix" to the end of both the file and the function.

That is, random_circulant_gram.py should be random_circulant_gram_matrix.py (and similarly for the function name). Updating subsequent docs and tests would also need to be done.