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/randomProcesses/noise/noiseModels/noiseModel/noiseModel.C

@@ -25,7 +25,7 @@ License
 
 #include "noiseModel.H"
 #include "functionObject.H"
-#include "stringOps.H"
+#include "argList.H"
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
@@ -143,7 +143,7 @@ Foam::fileName Foam::noiseModel::baseFileDir(const label dataseti) const
 {
     return
     (
-        stringOps::expand("<case>")    // ie, globalPath()
+        argList::envGlobalPath()
       / functionObject::outputPrefix
       / "noise"
       / outputPrefix_