Add operator+=

This simplifies updating residuals.
Expose PhaseUsage to clients

This is likely to be a hack, but it does facilitate creating the fully
implicit solver for arbitrary two- and three-phase fluid systems.
Add first cut at fully implicit system assembly.

Compile-tested only.

At this point, we only include reservoir effects--i.e., we don't
consider wells.
Temporarily ignore viscosity derivatives.
Clarify some variables' purposes.
Merge branch 'master' into fully-implicit
Renamed ImplicitBOStep -> FullyImplicitBlackoilSolver.
Added (empty) implementation file.
Class FullyImplicitBlackoilSolver is no longer a template.
Moved implementation to .cpp file.
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.
Added simulator class for fully implicit blackoil.
Merge remote-tracking branch 'atgeirr/fully-implicit' into fully-implicit
Created fully implicit simulator sim_fibo_ad.
Merge remote-tracking branch 'atgeirr/fully-implicit' into fully-implicit
Include well support in FIBOSolver interface.

Update callers accordingly.
Added Span.
Use Span.