ErtGrid: pass NULL handle to ERT if the MAPAXES data is garbage

this sometimes caused a segfault if the MAPAXES keyword was not
present in the grid. strangly enough, it also worked in some cases...
ECL well manager: adapt the well controls/completions before each report step
improve the ECL well model

the biggest change is that it is now based on a new approach: the well
model now always calculates the bottom hole pressure for the full well
when asked for a source term. This change makes it possible to
implement cross flow within wells properly and should also make the
well model physically correct.

Also, the well model now uses the connection transmissibility factor
which makes it possible to use this quantity if it is specified by the
deck...
discretizations: only make sure that the non-pseudo primary variables are defined

if Valgrind::CheckDefined() is used for the full primary variables
object, the padding (which is automatically introduced by the
compiler) causes valgrind to complain even if there is no issue...
ECL well model: handle the newish CMODE_UNDEFINED enum values
add the general framework for specifying auxiliary equations.

this does not disrupt the block nature of the linearized matrix
(i.e. Dune::BCRSMatrix is still used), but if the number of auxiliary
equations is smaller than that of the "main" discretization, the
superfluous equations are padded. if the number of additional
equations are larger than that of the equation, additional DOFs are
added.
ECL well model: make it an auxiliary module

i.e., the well equations are now considered in the linearization of
the global system of equations.

TODO: make it work for MPI parallel runs...