versatility improvement: #includeIfPresent dictionary directive

- similar to the #include directive, but does not generate an error if the
  file does not exist.

Note: opted for an explicit naming #includeIfPresent rather than #cinclude

applications/test/dictionary/testDict

@@ -78,6 +78,7 @@ boundaryField
 // NB: the inputMode has a global scope
 #inputMode merge
 #include "testDict2"
+#includeIfPresent "SomeUnknownFile"
 
 foo
 {