[Design] AtlasGroupStrategy

[mattrpav]

Discussed w/ the team the idea of an AtlasGroupStrategy that would be a pre-cursor to conditional logic, expressions and advanced Field Actions (including Math related on non-Collections).

The GroupStrategy would collect source values and have a FieldGroup collection of sorts to pass to FieldActions that support FieldGroup as an input would be responsible for creating the single value output.

Another thought was that the CombineStrategy could be converted to a Group-of-Field field Action and then deprecate the CombineStrategy in the engine.

The UI could then still have a first class mapping action called 'Combine', that would create a group strategy and apply a combine field action