Ensuring the service model's timestampFormat is correctly propagated to ...

Service's that have `timestampFormat` set in their respective model currently do not
propagate this to fields during the `stage1 -> stage2` transformation step. This leads the solver to incorrectly
assign the default `ISO8601` timestamp format to fields, if they have not explicitly been annotated with a format in the model.

This PR ensures the service's `timestampFormat` is used, if no format is specified at the field level.

> In reference to #13