Ensure (average of) tracers sum to one for DG1.
Add phaseUsage() method to BlackoilPropertiesInterface.

Also implement method in derived classes.
Always interpret 'ref_pressure' as a double

Otherwise, when specifying (e.g.)

   ref_pressure=1.0e-5

(1 Pascal in bars), the value gets reinterpreted as

   ref_pressure=1

which is one bar.
Merge pull request #241 from bska/double-refpress

Always interpret 'ref_pressure' as a double
Merge pull request #240 from atgeirr/tof-improvements

Ensure (average of) tracers sum to one for DG1.
Merge remote-tracking branch 'upstream/master' into add-phaseusage-to-interface
Merge pull request #244 from atgeirr/add-phaseusage-to-interface

Add phaseUsage() to BlackoilPropertiesInterface
Merge branch 'master' of git://github.com/OPM/opm-core