Add climate index tutorial notebooks

[tlogan2000]

Create step by step notebooks for a typical 'Portraits Climatiques' data analysis

• [x] * Finding and accessing datasets
• [x] * Spatio-temporal subsetting
• [x] * Climate index calculations
• [x] * Ensemble statistics & metrics
• [ ] * Data visualization

[tlogan2000]

Leaving open ... can add to finished check boxes as we add more notebooks