Adds -= operator
An elementwise -= operator is added to the autodiff class.
Adds new well formulation
Todo: incorporate WellDensitySegment. Currently values of the pressure
drop is hardcoded to make the rest of the code work
Todo: make it possible to shut perforation with crossflow.
Merge remote-tracking branch 'totto82/new_well_formulation' into new_well_formulation
Whitespace fix.
Minor whitespace adjustments.
Work in progress: use WellDensitySegmented class.

This is work in progress since it is non-working: phaseRates() is
not used correctly, and changes must be made to WellState or equivalent.
Merge branch 'master' into new_well_formulation

Conflicts:
	opm/autodiff/FullyImplicitBlackoilSolver.cpp

To resolve conflicts, WellState was changed to WellStateFullyImplicitBlackoil
in multiple places, and perfRate() changed to perfPhaseRate() in
WellDensitySegmented.
Merge branch 'master' into new_well_formulation
Fix sign bug in well contribution to mass balance.
Add currentControls() field to well state class.

This is done since the solver will need to be able to switch well controls
during Newton iterations. The current control specified in the Wells struct
will be used as default and initial value for currentControls().
Use currentControls() field of well state class.

Now the well state is consulted for the controls to use when assembling
the well control equations, instead of the (immutable) wells_ struct.

Also, added dummy implementation of updateWellControls().
FullyImplicitBlackoilSolver: make constantState() and variableState() consistent

With this, constantState() just calls variableState() and throws away
the derivatives. This might be a bit slower than necessary, but it
makes these two methods automatically consistent and constantState()
is only called once per timestep anyway...

thanks to @atgeirr for the suggestion!
Implemented updateWellControls().
Add end-of-line to switching message.
Fix bug in access order for state.qs.

State.qs is called well_phase_flow_rate in this function.

It is stored with wells running fastest, not phases.
Fix bugs relating to wells versus perforations.

Two kinds of bugs: correct usage of wops_.w2p vs. wops_.p2w, and
correct sizes for some variables (nw vs. nperf).
Merge branch 'master' into new_well_formulation

Needed to avoid compile errors with updated opm-core.
Fix bug in -= operator.
Removed dead code in test_block.cpp.
Added test for += and -= operators.