Represents a category as defined in syllabus.csv. A RefCell is used for
the scores field as it’s the only field that will need to be mutable once
an instance is created. In other words, the Vec of scores will be the only
thing that will need to be modified at runtime.
The syllabus for the course, containing the breakdown of course grade categories (the
percentage each category is worth) overall. Also keeps track of the number of categories.