Implement residualNorm()

This is a prerequisite to having a working Newton loop.
Merge remote-tracking branch 'atgeirr/fully-implicit' into fully-implicit

Conflicts:
	opm/autodiff/ImplicitBOStep.hpp
Implement residualNorm() for reservoir part.

This is a prerequisite to having a working Newton loop.

This is a forward-port of commit 604f866.
Merge remote-tracking branch 'atgeirr/fully-implicit' into fully-implicit
Merge remote-tracking branch 'atgeirr/fully-implicit' into fully-implicit
Include well support in FIBOSolver interface.

Update callers accordingly.
Merge remote-tracking branch 'atgeirr/fully-implicit' into fully-implicit
Add gather/scatter support for wells
Add an operator<< for outputting vector results.

Use it out print reservoir and well pressures at end of solution.
Merge remote-tracking branch 'bska/fully-implicit' into fully-implicit
Merge remote-tracking branch 'bska/fully-implicit' into fully-implicit
Renamed residual_.reservoir -> residual_.mass_balance.

Also initialize in initializer list, eliminating the need for
the allocateResidual() method.
Commented the assemble() function.
Added bhp as a primary variable.

Changed interface of constantState() and variableState() to also
take a WellState as input. Some comments added with minor layout
changes.
Added assembly of well equations.

This has not been tested.
Well contributions to mass balance equations are not done yet.