ENH: improve flexiblity for flat output of items (#1929)

- change to a templated implementation instead of relying on
  the container's writeList() method.

  This inlines the generation while also adding the flexibility to
  define different delimiters (at compile time) without the
  performance penalty of passing run-time parameters.

applications/test/flatOutput1/Make/files

@@ -0,0 +1,3 @@
+Test-flatOuput1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-flatOuput1

applications/test/flatOutput1/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */