ENH: simplify use of case-relative paths

- provide relativePath() for argList and for Time.
  These are relative to the case globalPath().
  Eg,

     Info<< "output: " << runTime.relativePath(outputFile) << nl;

src/functionObjects/field/streamLine/streamLineBase.C

@@ -786,7 +786,7 @@ bool Foam::functionObjects::streamLineBase::writeToFile()
         propsDict.add
         (
             "file",
-            scalarVtkFile.relative(time_.globalPath(), true)
+            time_.relativePath(scalarVtkFile, true)
         );
         setProperty(fieldName, propsDict);
     }
@@ -798,7 +798,7 @@ bool Foam::functionObjects::streamLineBase::writeToFile()
         propsDict.add
         (
             "file",
-            vectorVtkFile.relative(time_.globalPath(), true)
+            time_.relativePath(vectorVtkFile, true)
         );
         setProperty(fieldName, propsDict);
     }