Move member earlier in class to get more logical init

We want this member to be initialized directly after the fine properties
and the top surface has been set.
Add comment to help troubleshoot error

If the initial CO2 saturation is higher than 1 - Swr, we will get an
overflow; this is typically the reason for this assert so we add a
message here to help troubleshooting
Initialize phase position from density

Always assume that CO2 is the lightest phase, regardless of the position
this phase has (i.e. if it is the first or second phase in the deck)
Fix comment about location of brine pressure

The brine pressure was moved from the bottom to the top in commit
2aad605; the comment now seems to suggest otherwise because it wasn't
fixed at the time. Thus, delete the now erroneous part of the comment.
Add comment about tilting reservoir and gravity vector

In the equations, we write \nabla z \cdot \underline g, but here we
just use a scalar which is assumed to contain this inner product.
Return only a canonical cap. pres. and derivative

The core OPM code assumes that only the first of the phases have its
derivative set; the other entries must have the magic value zero in
order to pass an assertion.
Hoist constant to make shorthand

The compiler is able to hoist the constant product itself, but in this
case it is done not to optimize, but to create a shorter expression to
avoid unreadable code.
Merge pull request #43 from rolk/043_wat

Allow use of "water" phase for CO2