Improvements to combinations.py

[Rolv-Apneseth]

I have made some improvements to combinations.py. Pr #3681 (Forgot to make issue before submitting pr apologies)

Added:

• A description at the top, which includes a wikipedia link
• Type hints to the combinations function
• Some more doctests, which include 0 and negative numbers
• A check within the function so the function does not attempt to get factorials of negative numbers
• Some print statements in the main block which show some practical applications of the function

If you would like anything else, or if you don't like the changes, please let me know. Thanks!

[Rolv-Apneseth]

If this is ok, I can try to do the same for some other .py files which might need some extra touches like this