BanditScheduler: Add emitter_pool and active attr; remove emitters attr

[btjanaka]

Description

Currently, the BanditScheduler has an emitters attr that returns the _active_arr variable. This does not make much sense since BanditScheduler does not have emitters as input, and _active_arr gives emitters a different meaning from the emitters attribute of Scheduler.

Thus, this PR removes the emitters attr and instead adds emitter_pool and active to indicate the pool of emitters in the scheduler and which ones are currently active, respectively.

TODO

• Remove BanditScheduler.emitters
• Add BanditScheduler.emitter_pool
• Add BanditScheduler.active

Status

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

[btjanaka]

Will merge once #489 is resolved.