Updated surprise_utils.py

[zhaisw]

Description

Updated surprise_utils.py in order to increase predicting efficiency.
Previous code used the following to iterate users and items:

for user in data[usercol].unique():
        for item in data[itemcol].unique():
...

Updated to:

users = data[usercol].unique()
items = data[itemcol].unique()

for user in users:
        for item in items:
...

The update can avoid DataFrame unique operation in every for-loop step.

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging and not master.

[miguelgfierro]

cool, thanks!