Remove DQDEmitterBase

[itsdawei]

Description

Remove DQDEmitter base.

For emitters, I noticed that GaussianEmitter and IsoLineEmitter don't do anything in their tell() methods, and I also noticed it's a bit clunky to have DQDEmitterBase in the hierarchy. What if we made ask(), tell(), ask_dqd(), and tell_dqd() all have default behaviors of doing nothing (i.e. the ask methods return an empty array, and the tell methods do nothing). Then we could get rid of DQDEmitterBase because calling ask_dqd() and tell_dqd( on a given emitter would simply do nothing.

TODO

• [x] Remove DQDEmitterBase
• [x] Fix tests
• [x] Fix scheduler

Questions

• [ ] Should we have tell return None?

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go