Add initial support for unknown and custom types.

Add support for both custom and unknown custom types.  For now, all unknown
types are simply treated as reals for generation purposes.  Custom types can
be registered with a particular VariableParser using any previously existing
type.  All the generation code ignores the new type name and continues as
before.

git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_javization@11086 57e6efec-57d4-0310-aeb1-a6c144bb1a8b