Dimension Type Hierarchy Improvements

[RickMoynihan]

Moving suggestion from comment on issue #55 here.

We should improve the type hierarchy around dimensions, resources and dimension values.

As a first cut I'd suggest something like this (taken from comment), but we should discuss more:

interface Resource {
  uri: ID!
  label: String!
}

interface Dimension {
  uri: ID!
  label: String!
}

interface DimensionValue {
  uri: ID!
  label: String!
}

type DefaultDimension implements Dimension {
  uri: ID!
  label: String!
  values: [DimensionValue]  
}

type DefaultDimensionValue implements DimensionValue {
  uri: ID!
  label: String!
}

type EnumDimensionValue implements DimensionValue {
  uri: ID!
  label: String!
  enum_name: String!
}

type EmumDimension implements Dimension { 
  values: [EnumDimensionValue]
}

type HierarchicalValue implements DimensionValue { # i.e. could be a RefAreaValue
  uri: ID!
  label: String!
  children: [DimensionValue] # NOTE you can't have recursive datatypes in graphql :-( but we could potentially improve later by generating more specific types for each area level etc...
}

type HierarchicalDimension implements Dimension { # i.e. could be a RefAreaDimension
  values: [HierarchicalValue]
}