ENH: provide argList::envGlobalPath() static method

- this is identical to either of these solutions:

     * getEnv("FOAM_CASE")
     * stringOps::expand("<case>")

  but with a closer resemblance to argList or Time globalPath(),
  which makes the intent clearer.
  Avoids using raw strings in the caller, which improves compile-time checks.

  Used in situations where a class has no derivation path or other
  access to a time registry or command args.

src/sampling/sampledSurface/writers/boundaryData/boundaryDataSurfaceWriter.C

@@ -25,6 +25,7 @@ License
 
 #include "boundaryDataSurfaceWriter.H"
 #include "makeSurfaceWriterMethods.H"
+#include "argList.H"
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
@@ -62,7 +63,7 @@ Foam::fileName Foam::boundaryDataSurfaceWriter::write
     const pointField& points = surf.points();
 
     // Dummy time to use as an objectRegistry
-    const fileName caseDir(getEnv("FOAM_CASE"));
+    const fileName caseDir(argList::envGlobalPath());
 
     Time dummyTime
     (

src/sampling/sampledSurface/writers/boundaryData/boundaryDataSurfaceWriterImpl.C

@@ -54,7 +54,7 @@ Foam::fileName Foam::boundaryDataSurfaceWriter::writeTemplate
     const faceList&    faces = surf.faces();
 
     // Dummy time to use as an objectRegistry
-    const fileName caseDir(getEnv("FOAM_CASE"));
+    const fileName caseDir(argList::envGlobalPath());
 
     Time dummyTime
     (