1. Get dap4 case in catalog entry to work. This entailed:
   a. moving Dap4Servlet into tds (thredds.server) so
      so that DatasetHandler was accessible without
      causing a pom cycle
   b. Modifying Dap4Servlet to get the location of
      its files using DatasetHandler
   c. Modify servlet names Dap4->dap4
3. Modify TestDir to allow testing against various test servers.
4. Fix unwrap code to handle case where VariableDS has no base Variable
5. Modify the handling of enums to match Variable enums to the
   corresponding enum type decl. This is a bit of a flaw in
   HDf5 Iosp code.