headCount aggregate function for the Student controller

[djbowen95]

To meet this criteria from the brief:

• The Student controller should have a headCount aggregate function to get the total number of students by making use of MongoDB aggregate operators.

The brief asks for two aggregate functions so I should work on them together.

I think that these two 'tasks' on the list aren't really tasks - just notes to help build these aggregate functions:

• The project will require research of MongoDB operators such as $addToSet, $unwind, $group, $match, and $avg.
• Student lookup will require use of the ObjectId() method.

It is worth spelling out somewhere what each of them does in layman's terms - and probably want to use them all, or explain why I didn't use them all (even if it means solving the problem twice).

[djbowen95]

This is in the studentController() file