Added tutorial on running OGGM with avalanches

[MarinKneib]

Tutorial contains:

• a note on dependencies required to use Snowslide in its current version with OGGM
• an explanation of the avalanche maps in the new OGGM glacier directories
• an explanation of the informed 3-steps calibration with avalanches
• instructions to display the new mass balance gradients + how to make future projections of volume change accounting for avalanches
• a final note on using dynamical spinup with the monthly TI Avalanche model