problems: fix some rename-fallout for the postTimeStep() to endTimeStep() drive
Boundary contexts: add a numBoundaryFaces() method
element contexts: add a dofVolume() method

so far, only dofTotalVolume() was present...
FV base residual: make it consistently apply to only the primary DOFs

I hope...
models: make the globalStorage() method const
replace a few now-incorrect "Vcfv" prefixes in the comments
fix the parallel case for the new newton tolerance calculation

it was not considered that non-interior enties are expressed as having
a DOF volume of 0, which lead to an inf by calculating the square root
of zero...
parallelization: some smaller cleanups
parallelization: syncronize the solution after every Newton iteration instead of each time step

this lead to desyncronization of the ECFV discretization in the
parallel case.
parallelization: treat off-diagonal matrix entries on the process boundaries properly

this is required so that the element-centered finite volume method
does not handle each partition of the domain separately. (i.e. so that
fluxes accross faces on the process boundaries are considered) mea culpa!

The fix for this is to also include these entries in the matrix which
uses domestic indices. This required some rather extensive changes to
the blacklisting mechanism as for this it must be possible to
translate the index of a blacklisted entity (i.e., an entity in a
ghost or an overlap cell) to a domestic index (i.e., the corresponding
index in the algebraic overlap).

Note that the code for algebraic overlaps is *fun* and the person who
wrote it should be tarred and feathered. (*ouch* ;)) Seriously: Better
approaches than "lets-throw-this-away-and-use-grid-overlaps" are
deeply appreciated. (The grid overlap is not really useful in Dune
because only "Mickey Mouse grids" like Dune::YaspGrid support it.)
FV discretizations: add checkConservativeness() debugging method

this method checks that the difference in the storage terms before and
after a time step is the same as the accumulated fluxes over the
domain boundary plus the source terms.