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/test/spline/Test-spline.C

@@ -67,7 +67,7 @@ int main(int argc, char *argv[])
 
     bool useBSpline = args.found("B");
     bool useCatmullRom = args.found("CMR");
-    const label nSeg = args.opt<label>("n", 20);
+    const label nSeg = args.get<label>("n", 20);
 
     if (!useCatmullRom && !useBSpline)
     {