FIX: mfd_table and mfd_table_id resolver on InversionSolutionInterface return null

[chrisbc]

example:

query InversionSolutionDiagnosticContainerQuery($ids: [ID!]) {
  nodes(id_in: $ids) {
    result {
      edges {
        node {
          __typename
          ... on Node {
            id
          }
          ... on InversionSolutionInterface {
            created
            #mfd_table_id
            mfd_table {
              ... on Table {
                id
              }
            }
          }
          ... on InversionSolution {
            mfd_table_id
          }
        }
      }
    }
  }
}

variables: {"ids": ["SW52ZXJzaW9uU29sdXRpb246MTAwMDQ0", "SW52ZXJzaW9uU29sdXRpb246MTAwMDQx"]}

[chrisbc]

cl;osed by #228