Add interface for wet gas

The pvt interface is extened to handle wet gas
1. A function for rvSat is added to the interface
2. An interface that takes rv and the fluid condition as an input for
the gas properties is added. The old interface without rv and the fluid
condition is kept in the file.
3. The new interface is implemented in BlackoilPropsAd and
BlackoilPropsAdFromDeck.

A simulator that tests wet gas is not yet implemented.
Implementation of live gas
The simulator now handles live gas as well as live oil.
The primary variables are Po,Sw and Rs,Rv or Sg depending on fluid
condition
State 1 Gas only (Undersaturated gas): Po, Sw and Rv
State 2 Gas and oil: Po, Sw and Sg
State 3 Oil only (Undersaturated oil): Po, Sw and Rs

This commit includes:
1. New interfaces for the vapor oil/gas ratios (Rv)
2. Modifications in the equations to handle rvs
3. New definition of ADI variable to handle changing primary variables
4. Modifications in the solution updates to handle changing primary
variable
5. Some changes in the appleyard process to sync with Mrsts livegas
implementation.

NOTE:
The implementation is tested on the liveoil cases SPE1 and a simplified
SPE9 and produces the same results as the old code.
The simulator is not yet able to converge on SPE3 with livegas present.
For SPE3 to converge a more robust well implementation is needed. The
current simulator reproduce the results of Mrst when a similar well
model is used in Mrst as is currently implemented OPM.
[cmake,bugfix] Corrects required version and make Eigen3 download work.

At least on Debian requiring version 3.1 of Eigen3 is not sufficient
according to issue #25 the minimum version is 3.1.2. This patch fixes
the required version. In addition it skips the REQUIRED option passed
to find_package(Eigen3) to make the git download work.

Fixes #25.
Initial version of segmented well model.

This computes pressure differences with respect to the bottom-hole pressure
for each well perforation, based on the well flows. It is explicit and not
implicit, using the previous time step's flow rates to calculate the necessary
densities.
Added a test program for SegmentedWellModel.

The only currently tested case is a simple water injector.
Documented interface for SegmentedWellModel.
Fix mixture calculation for volatile oil case.
Modified test case to match MRST test case.
Bugfixes in treatment of perforation depths and final accumulation.
Fixed missing documentation
Fixed from bitwise to logical operator
The && is used instead of &
Fixed white spaces
Comment out unused code
Part of the code that is no longer in use is commented out.
Merge pull request #74 from totto82/wetgas

Implemention of live gas
Merge pull request #77 from blattms/fix-debian-eigen

[cmake,bugfix] Corrects required version and make Eigen3 download work.
Merge branch 'master' into seg-well-model
Renamed SegmentedWellModel->WellDensitySegmented.
Proper memorymanagement in test program.
Fix indexing error in if-test.
Use correct number of items in initialiser.