added elimination of special chars in ViewDefNameService
method renaming in TriplesMap

- declared getSubject method to be deprecated since I find it
  misleading: when calling triplesMap.getSubject() one could expect to get
  the subject map
- introduced a method named getResource being a copy of getSubject hoping
  that calling triplesMap.getResource() better reflects that the triples
  map resource will be returned
renamings in R2RMLSpecTest due to previously renamed method