use phase pressure to compute FVF.

That is strange, since in function ``fluidDensity()``, the phase pressure are passed to compute ``fluidReciprocFVF()``, but in function ``computeAccum()``, reference pressure still be used to compute FVF.
After this change, a simple case is tested and the result improves a little.
I didn't notice this before, but the above indexing is wrong.  Method `computePressures()` returns a value for each global phase, not just the active ones.  Therefore we must use
```
pressures[phase]
```
rather than
```
pressures[pos]
```
in the call to method `fluidReciprocFVF()`.