Make multiphase modeling easier

[jgostick]

This has been bugging me for a while, and now I'm doing to multiphase calcs, so I will try to capture the changes we need:

1. Perhaps Phase objects should have 'occupancy' initialized by default? The multiphase conductance models need these to run (but don't care if they are all zeros).
2. The occupancy of the first phase could be all 1's, but then all subsequent phases be 0's
3. As the occupancy of a Phase changes, the other phases could be automatically adjusted for consistency
4. Item 3 might not make sense in simulation with Phases that don't interact (like if there is a 'solid' phase), so perhaps this is a good junction to start thinking about 'mixture' objects again?

[jgostick]

We have added a MultiPhase class that does this...sort of. I will close this on the basis that we'll be fleshing out the multiphase class over the next little while and the issue should be more clearly understood then.