COMP: drop gcc-4.2 settings

Added compressibility support to MRFZone.
Required generalisation of the oneField concept.
Also clean-up the use of one and zero for consistency.
Simple mixerVessel2D tutorial case supplied.

- the path is accesible either way, but use setenv for consistency with
  other environment variables. It also simplifies foamCleanPath again.

For example, within the user ~/.OpenFOAM/<VER>/prefs.sh:

    foamPrefs=`$WM_PROJECT_DIR/bin/foamEtcFile -m go prefs.sh` \
        && _foamSource $foamPrefs

could be used to write a personal adjustment that also includes the
site-wide prefs.

BUG: compilerInstall was getting overwritten in settings.{csh,sh}

non-numeric (eg, a git or cvs version).

- place the onus on the user to provide the correct ParaView_MAJOR in
  these situations.

STYLE: Drop support for cmake older than 2.6.4 (installed in 3rdParty)