Multivariable questions super PR

[cameron-toy]

What's New?

This PR combines several commits from previous PRs.

New:

• Support for multivariable questions! QA objects for questions involving multiple properties can now be automatically generated and answered ([PROF]'s office hours are [PROF..OfficeHours] in [PROF..Office]).
• New Prof entity type. This entity replaces the existing Professors entity and is a combination of relevant information from the Professors, Sections, and OfficeHours entity. This unified entity allows answering many more fact-based questions than before.
• QA objects now take a database parameter during initialization, removing top-level initialization of the database in QA.py
• New QA functions for chaining database access, formatting professor office hours, and more.

Old (but still new):

• NLP performance increases.
• Removal of nltk and spacy as dependencies.
• Replaced NIMBUS_NLP.py with variable_extractor.py
• Added (currently unused) ProfessorSectionView entity.
• Replaced q_a_pairs.csv with @richagadgil's get_all_answerable_pairs function. Note: function is no longer used directly in question_classifier.py. See build_question_classifier for explanation.

Type of change (pick-one)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

See PRs #110 and #103. Also, multivariable question from database answered in if name==... block in nimbus.py.

Possible concerns:

• Slightly different overall features generated by spacy and spacy's stopwords list.
• Longer database access times for views.

TODO:

• Type annotation

Checklist (check-all-before-merge)

formatting help: - [x] means "checked' and - [ ] means "unchecked"

• [ ] I documented my code according to the Google Python Style Guide

• [ ] I ran ./build_docs.sh and the docs look fine

• [ ] I ran ./type_check.sh and got no errors

• [x] I ran ./format.sh because it automatically cleans my code for me 😄

• [ ] I ran ./lint.sh to check for what "format" missed

• [ ] I added my tests to the /tests directory

• [ ] I ran ./run_tests.sh and all the tests pass

[cameron-toy]

I’ll try to get a version of this PR passing style checks and type annotations up by this weekend.