Take into account MARC relator codes in RDA entries

[acka47]

Sub-issue of https://github.com/hbz/lobid/issues/161, see https://github.com/hbz/lobid/issues/161#issuecomment-161566116.

We just have to take the code from subfield 4 in the 1* and 2* fields and append it to http://id.loc.gov/vocabulary/relators/ to get the relator code for the respective entry. Default entry (at least for person) is aut but for now we will continue using dct:creator.

Examples:

http://lobid.org/resource/HT018772877, source (snippet):

<datafield tag="100" ind1="b" ind2="1">
  <subfield code="p">Klein, Volkmar</subfield>
  <subfield code="4">edt</subfield>
  <subfield code="3">Herausgeber</subfield>
  <subfield code="9">(DE-588)174630727</subfield>
</datafield>

http://lobid.org/resource/HT018776405, source (snippet):

<datafield tag="200" ind1="b" ind2="1">
  <subfield code="k">Theater im Pfalzbau</subfield>
  <subfield code="4">isb</subfield>
  <subfield code="3">Herausgebendes Organ</subfield>
  <subfield code="9">(DE-588)301289-X</subfield>
</datafield>

https://lobid.org/resource/HT018781534, source (snippet):

<datafield tag="100" ind1="-" ind2="1">
  <subfield code="p">Schroeder, Hermann</subfield>
  <subfield code="d">1904-1984</subfield>
  <subfield code="4">cmp</subfield>
  <subfield code="3">Komponist</subfield>
  <subfield code="9">(DE-588)118823442</subfield>
</datafield>

https://lobid.org/resource/HT018782520, source (snippet):

<datafield tag="104" ind1="a" ind2="1">
  <subfield code="p">Heitmann, Michaela</subfield>
  <subfield code="d">1969-</subfield>
  <subfield code="4">art</subfield>
  <subfield code="3">Künstler</subfield>
  <subfield code="9">(DE-588)142552739</subfield>
</datafield>

[acka47]

@dr0i Is the goal clear or should I add JSON snippets for expected outcome?

[acka47]

Moved to https://github.com/hbz/lobid-resources/issues/72 as this will be dealt with in API 2.0. Closing.