Support GBIF queries from `galah`

[mjwestgate]

In a previous issue (#126) we expanded support to all available living atlases. We currently (v 1.5.0) support the GBIF name-matching service for some atlases, but no other services; I suspect this is a strange oversight to most users.

The first set of challenges to upgrading to GBIF are:

• No GBIF API for field names, assertions etc (rgbif hard codes them)
• Need for new translators in galah_filter as GBIF doesn't use solr
• The 'predicates' system for GBIF downloads is analogous to our solr approach (albeit in JSON), but the 'online' GBIF occurrence API doesn't use them, potentially creating a mismatch between atlas_counts (online) and atlas_occurrences (offline)

Other problems may emerge once these are solved. However, the two systems are otherwise closely related, so it is conceptually straightforward to map current galah functions to GBIF.

[mjwestgate]

Closing this with release of v1.5.1