Resolve "Align fgen with domain model"

Description

• remove the use of the name calculator entirely. These things are just objects (sometimes they're models, other times they're other things, but calculator was the wrong name to use)
• CalculatorDefinition -> DerivedTypeDefinition
• LinkDefinition -> ModuleDependencyDefinition
• fgen.models -> fgen.data_model to make clear that this is fgen's data model (to avoid confusion with 'model', which generally refers to a physical model in what we do)
• model -> instance generally e.g.
  • base finalizable.model_index -> base finalizable.instance_index
  • model_avail -> instance_avail
  • calculator_manager -> instance_manager etc.
  • model -> instance in fgen_runtime exceptions

Checklist

Please confirm that this pull request has done the following:

- [ ] Tests added

• Documentation added (where applicable)
• Changelog item added to changelog/

Closes #53 (closed) #50 (closed)