ACI/NH: rework code to add snow contribution

[ikalash]

For imposing the right BC to incorporate the impact snowpack on top of the Paulatuk runway, we need to add the capability to incorporate snow effects. Denser snow is expected to be a worse insulator. Proposed approach:

• We currently have 3 phases in mixture model: ice, water, soil.
• Add 4th phase, air. This will be 0 for most materials - all but snow. Snow = air + ice + water.
• Read in volume fraction of 4 materials: ice, water, soil, air.
• Thermal conductivity and heat capacities of snow will be calculated from volume fractions (?)

[ikalash]

This has been done in the add-snow-air-props branch. Closing.