ENH: additional text expansion shortcuts (issue #792)

Support the following expansions when they occur at the start of a
string:

    Short-form       Equivalent
    =========       ===========
      <etc>/          ~OpenFOAM/   (as per foamEtcFile)
      <case>/         $FOAM_CASE/
      <constant>/     $FOAM_CASE/constant/
      <system>/       $FOAM_CASE/system/

These can be used in fileName expansions to improve clarity and reduce
some typing

     "<constant>/reactions"   vs  "$FOAM_CASE/constant/reactions"

tutorials/incompressible/pimpleFoam/RAS/TJunctionFan/0.orig/p

@@ -28,7 +28,7 @@ boundaryField
         direction       in;
         readerType      openFoam;
         hasHeaderLine   true;
-        file            "$FOAM_CASE/constant/FluxVsdP.dat";
+        file            "<constant>/FluxVsdP.dat";
         //nonDimensional  true;
         //rpm             300;
         //dm              2e-2;