Support number spans with additional spaces

[nichtich]

RVK notations have spaces around the - of number spans, e.g:

  <datafield tag="153" ind1=" " ind2=" ">
    <subfield code="a">MC 7700</subfield>
    <subfield code="c">MC 7773</subfield>

should result in notation MC 7700 - MC 7773 instead of MC 7700-MC 7773. I added a test case at https://github.com/gbv/mc2skos/tree/rvk-span/examples but no fix so far. This is likely yet another configuration of one particular KOS.

P.S: I'm not sure whether URIs should also have _-_ instead of -, need to check back at UB Regensburg.