Ion chamber timeout

[canismarko]

This PR adds a proper timeout calculation to the IonChamber.trigger() method. Previously, the default timeout was used regardless of the settings on the scaler. Now, the new method IonChamber.default_timeout() is used to determine the best timeout based on the scaler configuration.

Things to do before merging:

• [x] add tests
• [x] write docs
• [x] update iconfig_testing.toml
• [x] flake8, black, and isort