Provide DTD schema to describe parameter file format

Notice that this definition specifies a superset of the actual format
as the ParameterGroup tag is context-dependent (which cannot be captured
by DTD schemata).
Ensure (average of) tracers sum to one for DG1.
Add phaseUsage() method to BlackoilPropertiesInterface.

Also implement method in derived classes.
Merge pull request #238 from rolk/238_dtd

Provide DTD schema to describe parameter file format
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
Make implementation class "package" visible

Since a definition of the class is not in the header, no other
compilation units can safely access the internals of the Impl class
anyway, so it may as well be a regular struct so that the outer class
(SimulatorIncompTwophase) can access it.
Merge pull request #240 from atgeirr/tof-improvements

Ensure (average of) tracers sum to one for DG1.
Notify callbacks when a timestep has completed

Client code can do additional processing for the output of each
timestep.
Subtract time spend in callbacks from total

Since this is not time spent on the simulation itself, it is "unfair"
that this is accounted on the simulator.
Merge remote-tracking branch 'upstream/master' into add-phaseusage-to-interface
Provide template functions in separate header

Template functions must be defined in the header since the library
cannot contain generic code. To keep only the interface in the main
header, all such function bodies are put in a separate _impl file.
Stub out an abstraction of a Restart file.

Intended to encapsulate ecl_*() operations in function writeECLData().
Try singular name if plural not defined

In CMake "canon" (i.e. tutorial on Kitware's website) it is stated that
the _DIR suffix should be used for the directory of the library alone,
whereas _DIRS are entire list of paths necessary to compile (i.e.
dependencies are included).

Eigen3 only uses the singular form (since it doesn't have any
dependencies); it should really be copied to the plural form, but it may
be advantageous to have the find-module included unchanged, so we put in
an extra effort to find it here.
Merge pull request #244 from atgeirr/add-phaseusage-to-interface

Add phaseUsage() to BlackoilPropertiesInterface
Include possibility that uppercase is used
Merge pull request #243 from rolk/243_single

Try singular name if plural not defined
Merge branch 'master' into devel
Merge pull request #242 from rolk/242_callback

Let external objects observe individual timesteps in a simulation
Merge from upstream