Always select primary keys

[Migushthe2nd]

Fixes #9

This PR makes it so that the primary columns are always selected.
This will prevent joined relations from becoming null if no table column is selected

[Migushthe2nd]

This PR requires additional work in order to support for example __typename. Since it is not included in the tree.fields, the join (previews below) is added, but no field is selected.
An example query would be

query {
  layout(id: "db") {
    previews {
      __typename
    }
  }
}

[Migushthe2nd]

Additional check was implemented. It will now check in the EntityMetadata if a field is a relation, instead of checking if field.fields.length > 0. This fixes __typename.