ENH: add alternative STL ASCII parsers

- In addition to the traditional Flex-based parser, added a Ragel-based
  parser and a handwritten one.

  Some representative timings for reading 5874387 points (1958129 tris):

      Flex   Ragel   Manual
      5.2s   4.8s    6.7s         total reading time
      3.8s   3.4s    5.3s         without point merging

etc/controlDict

@@ -121,8 +121,12 @@ OptimisationSwitches
 
     // Force dumping (at next timestep) upon signal (-1 to disable) and exit
     stopAtWriteNowSignal    -1;
+
+    //- Choose STL ASCII parser:  0=Flex, 1=Ragel, 2=Manual
+    fileFormats::stl 0;
 }
 
+
 /* Can specify fallback profiling settings
 profiling
 {