restructure the directory structure of the implicit models

this is somewhat similar to what is currently done in the 'implicit'
branch of dumux, but it takes things further:

- ewoms/boxmodels/common -> ewoms/disc/vcfv
  (vcfv == vertex centered finite volumes)
- ewoms/boxmodels/vtk -> ewoms/vtk
  (possibly this will move to ewoms/io/vtk, TBD)
- ewoms/boxmodels -> ewoms/models
- test/boxmodels -> test/implicit
- the files for the common VCFV code have been renamed from "box*" to
  "vcfv\1" and the classes they contain have been renamed accordingly.

The rationale is to reflect the fact that multiple discretizations may
be chosen for any physical model. (Once these discretizations are
implemented, obviously.) All tests still compile for the autotools and
cmake build systems

examples/tutorial_coupled.cc

@@ -20,7 +20,7 @@
 /*!
  * \file
  *
- * \brief Main file of the tutorial for a fully coupled twophase box model.
+ * \brief Main file of the tutorial for a fully coupled twophase VCVF discretization.
  */
 #include "config.h" /*@\label{tutorial-coupled:include-begin}@*/
 #include "tutorialproblem_coupled.hh"  /*@\label{tutorial-coupled:include-problem-header}@*/

examples/tutorialproblem_coupled.hh

@@ -26,7 +26,7 @@
 #define EWOMS_TUTORIAL_PROBLEM_COUPLED_HH    // guardian macro /*@\label{tutorial-coupled:guardian2}@*/
 
 // The numerical model
-#include <ewoms/boxmodels/immiscible/immisciblemodel.hh>
+#include <ewoms/models/immiscible/immisciblemodel.hh>
 
 // The chemical species that are used
 #include <ewoms/material/components/simpleh2o.hh>
@@ -52,7 +52,7 @@ class TutorialProblemCoupled;
 
 namespace Properties {
 // Create a new type tag for the problem
-NEW_TYPE_TAG(TutorialProblemCoupled, INHERITS_FROM(BoxImmiscibleTwoPhase)); /*@\label{tutorial-coupled:create-type-tag}@*/
+NEW_TYPE_TAG(TutorialProblemCoupled, INHERITS_FROM(VcfvImmiscibleTwoPhase)); /*@\label{tutorial-coupled:create-type-tag}@*/
 
 // Set the "Problem" property
 SET_PROP(TutorialProblemCoupled, Problem) /*@\label{tutorial-coupled:set-problem}@*/