Cleanup model and add interface.

[ikeough]

BACKGROUND:
Some users of Elements have asked for Model to implement an interface (see #694).

DESCRIPTION:
This PR introduces the IModel interface and implements the interface in the existing Model class. It also refactors model extension methods into a separate file and cleans up unused namespaces (which will pay dividends in the transition to System.Text.Json).

TESTING:
No code was changed.

REQUIRED:

• All changes are up to date in CHANGELOG.md.

COMMENTS:

• Any other notes.

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This change is 

[andrewheumann]

LGTM, one stray comment

Reviewed 3 of 4 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @ikeough)

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Elements/src/Model.cs, line 59 at r2 (raw file):

        }

        /// <summary>O

O?