Add the section aggregate endpoints to the professor

[mikehquan19]

This pull request is for task #232. I added the section aggregate endpoints to the professor controllers. I also made the function getProfessorQuery(flag string, c *gin.Context) which gets the query of the professor depending on the flag that can be used for both professorCourse() and professorSection(). That way we can reduce redundancy in the code, though I think there's still lots of redundancy in my code.

Thank you.

[jpahm]

Awesome, thank you! Should be able to review this one much quicker than the last one :)