Ensure codegen script accommodates all subgraph types (eg derived fields)

[SvenMuller95]

• research subgraph types (currently 6 types)
• ensure tool is accommodating for all

[MJYoung114]

need to handle derivedFrom types -> suggestion is to ignore these in the generated code for an entity, The field will be set in the derived from entity https://thegraph.com/docs/en/developing/creating-a-subgraph/#reverse-lookups

[MJYoung114]

• Arrays of entities
• arrays of types
• bigDecimal

[JasoonS]

I think in the process of doing this task more type documentation and structuring can be done in the code. Currently almost everything is polymorphic. It might be good to use less polymorphic structures in the code.