ENH: allow "<case>", "<system>" ... in the string expansions (issue #792)

- the expansions were previously required as slash to follow, but
  now either are possible.

    "<case>", "<case>/" both yield the same as "$FOAM_CASE" and
    will not have a trailing slash in the result. The expansion of
    "$FOAM_CASE/" will however have a trailing slash.

- adjust additional files using these expansions

src/functionObjects/field/externalCoupled/externalCoupled.H

@@ -86,7 +86,7 @@ Usage
         type            externalCoupled;
         ...
         log             yes;
-        commsDir        "${FOAM_CASE}/comms";
+        commsDir        "<case>/comms";
         initByExternal  yes;
         stateEnd        remove;  // (remove | done)