Better handling of start_time = None and end_time = None

[NoraLoose]

This PR addresses two issues:

• start_time , end_time can be set to None #239
• Document bypass_validation argument and add to YAML #231

To fix #239, the following changes are made:

• BoundaryForcing and SurfaceForcing can still receive start_time = None and end_time = None, however a warning is triggered in that case. This option is now also documented in the docstrings
• InitialConditions cannot receive ini_time = None, an error will be raised in that case

To fix #231, the bypass_validation parameter is now written to YAML and read from YAML.

• Closes start_time , end_time can be set to None #239, closes Document bypass_validation argument and add to YAML #231
• Tests added
• Passes pre-commit run --all-files
• Changes are documented in docs/releases.md
• New functions/methods are listed in docs/api.rst