Schedulers warn if no solutions are inserted into archive

[btjanaka]

Description

Solutions may not be inserted into the archive if the threshold_min is too high. This can lead to unexpected behavior. For instance, GaussianEmitter with an initial set of solutions will keep producing that same set of solutions, and archive metrics like obj_max will be None. We shouldn't throw an error when this happens as users may decide they want this, but a warning is nice.

TODO

• [x] Warn in Scheduler
• [x] Warn in BanditScheduler
• [x] Test Scheduler warning
• [x] Test BanditScheduler warning

Questions

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