Misc. updates to gold_translator.py

[sujaypatil96]

As a result of the discussion on microbiomedata/issues#527 a couple of trivial edits needed to be made to the GOLD-to-Mongo ingest pipeline:

• Use lowercase gold while making CURIEs for GOLD entities
• Modification to the logic in _get_quantity_value() to accommodate special handling of depth data

NMDC depth.has_minimum_numeric_value — GOLD depthInMeters
NMDC depth.has_maximum_numeric_value — GOLD depthInMeters2

depthInMeters2 may or may not be present in GOLD, so in the cases where it’s not provided:
NMDC depth.has_numeric_value — GOLD depthInMeters