[feature] Enable default settings for TrainerSettings

[ervteng]

Proposed change(s)

Based on feedback from CarryCastle. Enables specifying a default TrainerSettings for all behaviors. The default is specified using the same structure as the others under a new header default_settings. These are now used as the default values for all behaviors, including those not found in the settings YAML.

default_settings:
    trainer_type: ppo
    ... etc.
behaviors:
    CrawlerStatic:

For behavior names that are defined in the YAML, any missing fields will be filled first from the default_settings, then those defined in the TrainerSettings class insettings.py.

WIP: Tests, documentation

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

[chriselion]

I'm not sure if this will do what you want for nested dictionaries. For example if you had

{"foo": {"x": 1}}.update({"foo", {"y": 2}})

you'd end up with {"foo", {"y": 2}}, but I think you really want {"foo", {"x": 1, "y": 2}} (i.e. recursively merge dictionary values)

[ervteng]

Fixed using a recursive dict update

[chriselion]

Suggested change

	Behaviors found in the environment that aren't secified in the YAML will now use the `default_settings`,
	Behaviors found in the environment that aren't specified in the YAML will now use the `default_settings`,