ENH: support 'get()' for retrieving argList options

- previously only had 'opt<..>()' for options, but 'get<..>()'
  provides more similarity with dictionary methods.
  The 'opt<..>()' method is retained.

applications/utilities/mesh/manipulation/createPatch/createPatch.C

@@ -534,7 +534,7 @@ int main(int argc, char *argv[])
     #include "createTime.H"
 
     const word meshRegionName =
-        args.opt<word>("region", polyMesh::defaultRegion);
+        args.get<word>("region", polyMesh::defaultRegion);
 
     const bool overwrite = args.found("overwrite");