Clean up code

[darkterbear]

Description of changes:

• Moved imports to the top
• Added another static "constant" 17 for rewards ratio for purchases above 250
• Added an error log to track erroneous LogEntries as specified by spec
• Use one-line unpacking to parse LogEntry
• Streamline LogEntry processing, reduce code redundancy
• Removed unnecessary get_items_purchased method

Which files were touched:

• rewardsPointsSystem.py

Notes for PMs:

• Would be nice if there was a method in defaultdict that lets me "upsert" an item to make code more clean/readable, but couldn't find anything

Relevant Issue:
#1

[kelvinjue]

Good work! We'll go over this with the rest of the team some time later this week.