Synchronise Initialisation, Saturation Functions and PVT Models with MRST and ECLIPSE

This is a set of changes that collectively bring feature and result parity with ECLIPSE and MRST with respect to initialisation, saturation functions, and PVT models. With these changes SPE1 and a simplified SPE9 without capillary pressure and with only two wells matches ECLIPSE and MRST reasonably well.

Specifically,

* The default three-phase oil relative permeability is changed to mirror those of ECLIPSE and MRST.

* Add a function that computes saturations from surface volumes.  This is used to compute initial saturation fields that honour initial mass conservation.

* Cases for which the calculated dissolved gas-oil ratio equals the **tabulated** ratio are now judged as saturated. This is important as the gas resolution factor is set to equal for saturated cases in the Appleyard process.

* The `initBlackoilSurfaceVolume()` that computes surface volumes based on the `Rs` factor is renamed to `initBlackoilStateFromDeckUsingRS()` to avoid confusion with the original function `initBlackoilSurfaceVolume()` that computes surface volumes based on saturations.

* Introduce a black-oil specific facility, class `PhasePresence`, to allow clients (i.e., simulators) to actively decide whether or not the conditions in a particular cell represent fully formed free gas ("saturated") phase in an O/G or W/O/G system.


This PR incorporates, supersedes and closes #434.