Add section type grade aggregation for Trends

[mikehquan19]

Hi! This is the commit for the #242. I basically modified the existing stages and add new stages so that the pipeline will split the semester into each section type. The code itself is messy, and hardcoded, so let me know if there's any limitations I can improve. I have a few questions:

• Can we modularize the code in the sense that we can write a separate function to return the appropriate pipeline? I think that would make the code more readable.
• Can we write the unit test to test this? For instance, the one that comes in my mind is all the sections in a semester of /grades/semester/section_type should add up to the same semester of /grades/semester.

[mikehquan19]

I've made the commit "Modify the function further to make the code more readable", in which I've tried my best to make the code more readable, shorter. Let me know if there's any problems.

A few notes: I've taken advantage of the fact that bson.D is technically a slice of bson.E alot.