Implement Atlantis functional group parser

[Bai-Li-NOAA]

No Atlantis equivalent exists for functional group standardization. Update get_functional_groups so it parses functional groups for Atlantis in the same way it does for EwE.
Tasks:

• Read Atlantis functional_groups.csv.
• Build species and group fields.
• Generate unique snake_case field.

Acceptance Criteria:

• Output has required four columns.
• Duplicate names are detected and error clearly.
• Tests cover common and edge naming patterns.
  Standardize Atlantis truth list into ecosystemom long format