Update gitignore so setup.py develop artifacts are ignored
CLIMATE-519 - Rework ESGF integration module structuring

The ESGF module is now part of the OCW package.
CLIMATE-519 - Rework ESGF helper module import structuring

Update ESGF module imports to reflect new package/module structuring.
CLIMATE-520 - Add ESGF module to setup.py
CLIMATE-518 - Add initial ESGF data_source
CLIMATE-518 - Add documentation to ESGF data_source
CLIMATE-518 - Add ESGF data_source to Sphinx build
CLIMATE-518 - Add descriptive default search URL constant
CLIMATE-518 - Add multi-dataset handling

- Add multi-dataset handling to the ESGF data source. Loaded dataset(s)
  are returned as a list now.
- Add support for elevation layer selection when loading a dataset.
CLIMATE-518 - Documentation Clarification
CLIMATE-538 - Add optional parameter for elevation level

- Add an optional parameter for selecting elevation level when loading a
  dataset. By default it is set to 0 (which is the current value used)
  to ensure backwards compatibility.
Resolve CLIMATE-538. Merge PR #118.
Resolve CLIMATE-518. Merge PR #115.