Manual only trigger refactor

[cicdw]

Closes #83 w/ a new PAUSE signal

Also separates out never_run and manual_only: never_run now raises DONTRUN and is a more aggressive signal than manual_only, which raises PAUSE causing inputs to be cached in the task_runner.

Additionally, adds a command line option to pytest: pytest --airflow will run the airflow tests (assuming you have airflow installed), otherwise they are skipped by default. These tests are slow and it was kind of annoying on my machine to have them run all the time.

[jlowin]

+1 for using a signal to indicate the flow should pause instead of parsing the exception for manual_only, but I'm not following the use case for having both manual_only and never_run as two separate triggers. Should we just get rid of one of them entirely?

Second, should --airflow flag be provided in CI to run all tests?

[cicdw]

Re: never_run: I'm not quite sure I had a use case in mind honestly. If that adds unnecessary granularity, happy to remove it and / or go back to aliasing it to manual_only.

For the --airflow flag, we could but those tests really take a long time (2.5 minutes vs. 22secs currently) and also would require us to install airflow at some point. I think at this moment I vote we leave it as is, but maybe when we start thinking about nightly test runs (integration tests w/ server) we can include these tests. Willing to include them though, but

[jlowin]

Let's kill never_run -- it's not a particularly useful name.

[jlowin]

PAUSE is a cool signal. Interestingly, it could be raised by any code, not just the trigger. That could be a very powerful extension.