method to get a dictionary of {user_pair:previous meeting count}

[harshk360]

• Renamed get_previous_pair_meetings to get_previous_meetings
• Moved get_previous_meetings (get_previous_pair_meetings) to a new match_utils.py file
• Added a method to convert a list of user pairs to a dictionary of pairs to counts (for how many times they have met)
• Added method to return a dictionary of {user_pair:previous meeting count} for a group of users to be matched
• Created a new file group_match.py with the above method
• group_match.py will hold the new group matching algorithm which will use the above method