Externalize subparameter calls and simplify Parameter copy

[jrapin]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context / Related issue

This untangles Dict from Parameter, by removing the assumption that all parameters have a _parameters field.
It adds a helper to deal with function propagation to subobjects.
It also simplifies the handling of copies, which now use a shallow copy and updates the required fields, so that it becomes way simpler for all sub-classes

How Has This Been Tested (if it applies)

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document and completed the CLA (see CLA).
• All tests passed, and additional code has been covered with new tests.