Added template notebook to analyze ONI and Nino3.4 Variability

GFDL pp-dir and catalog builder support

- Added func to infer if a string is a GFDL pp-dir
- Added check to see if GFDL pp-dir exists and is valid
- Implemented a top-level site check to see if esnb
  is running on a GFDL host
- Added func to infer GFDL experiment names
- Added subprocess call to GFDL catalog builder

Notebook Engine

- functions to clear, execute, and save esnb notebooks

Initial support for pangeo-cmip datasets

- added a template notebook
- added a try-block when deep copying source catalog

Propogate `z_i` as a coordinate if available

- If the variable `z_i` (ocean interfaces) is present
  in the source dataset, propogate it along as a coordinate

Functionality to read / write cached datasets

Initial working Pangeo support

- Added routine to infer source data file type
- Reworked catalog resolving logic
- Added function to open google cloud storage item

Updated `enso.ipynb` example notebook

Fixed linter issues

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