Update fws_occ function to pull occurrence data from ServCat

[timothymfox]

The fws_occ function uses the public facing ServCat API to extract taxonomic names for the given refuge using the unit code. The records are pulled from ServCat using the following constraints:

• The record includes the bounding box for the given refuge
• The record contains no other bounding boxes associated with other properties
• The record is one of the following: Book Chapter, Conference Proceeding, Conference Proceeding Paper, Geospatial Dataset, Journal Article, Published Report, Published Report Section, Published Report Series, Resource Brief, Tabular Dataset, or Unpublished Report
• The record is associated with at least one digital file