ENH: improve HashTable iterator access and management

- provide key_iterator/const_key_iterator for all hashes,
  reuse directly for HashSet as iterator/const_iterator, respectively.

- additional keys() method for HashTable that returns a wrapped to
  a pair of begin/end const_iterators with additional size/empty
  information that allows these to be used directly by anything else
  expecting things with begin/end/size. Unfortunately does not yet
  work with std::distance().

  Example,
     for (auto& k : labelHashTable.keys())
     {
        ...
     }

applications/test/cplusplus1/Make/files

@@ -0,0 +1,3 @@
+Test-cpluplus1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-cpluplus1

applications/test/cplusplus1/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = -I$(LIB_SRC)/cfdTools/include */
+/* EXE_LIBS = -lfiniteVolume */