various cleanups

- make all headers in dumux/common and dumux/boxmodels/common self sustained,
  i.e. that they can be included without any preconditions
- remove most irrelevant includes in the common code
- bring the 1p and 1p2c box model up to stable standard (accessor functions in
  the volume and flux variable classes, make the 1p2c model work for
  compressible fluids, various fixes the 1p2c test problem)
- change the isfluid-trail fluid system to standard SI units
- play around with timestep control if timestep ramp-up is enabled
  (problem is still not solved, though)
- split the *properties.hh headers into the declaration of the properties and
  definition of defaults for the common part of the box models as well as for
  the 1p and 1p2c models
- put the indices structure for the 1p and 1p2c models into separate files
- introduce a base class for volume variables of the box models

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4180 2fb0f335-1f38-0410-981e-8018bf24f1b0