add mappings for bd6xx.py

[switowski]

• [ ] Add mappings (based on documentation from: https://www.loc.gov/marc/bibliographic/)
• [ ] Add test examples (for each mapping, take the example data from LOC website. For example, if you go to this page: https://www.loc.gov/marc/bibliographic/bd383.html you will see lines like:

383 ##$ano. 14,$bop. 27, no. 2
383 ##$cBWV 211
383 ##$bop. 8, no. 1-4
, etc.

The idea is to convert those lines into MARCXML example data (don't do this by hand, write a script for that !), so we are sure all mappings are tested:

<record>
  <datafield tag="383" ind1=" " ind2=" ">
    <subfield code="a">no. 14</subfield>
    <subfield code="b">op. 27, no. 2</subfield>
  </datafield>
  <datafield tag="383" ind1=" " ind2=" ">
    <subfield code="c">BWV 211</subfield>
  </datafield>
  <datafield tag="383" ind1=" " ind2=" ">
    <subfield code="b">op. 8, no. 1-4</subfield>
  </datafield>
, etc.
</record>