use roles to define multi-use aspect of materials

[wdduncan]

Many restoration materials can be used in different ways, e.g., resin can be used as either an direct or indirect restoration. The current approach has been define processes that define how materials are used in a restoration procedure (e.g., see direct dental material insertion process or dental material tooth attachment process).

If we want to account for other uses of materials (e.g., as luting agents see #102 ), we can continue with the current approach, or we can add roles (realized during processes) to represent how a material is being used.

This change would require refactoring (though not a ton of it), but we should think of standardized design patterns for using roles, use cases that make more sense using roles, and difficulties that may arise from this approach.

cc @gopi-kris @addiehl