Task: Remove std::map inheritance, clean up interface

- Make the Task object's interface more explicit by removing the
  std::map inheritance.
- Using this more explicit interface, remove unneeded ctors in order to
  allow the compiler to "Do The Right Thing"(tm).
  This leads to a performance improvement of 12% in the "add"
  performance test, and 7% for "import".

src/commands/CmdUDAs.cpp

@@ -137,7 +137,7 @@ int CmdUDAs::execute (std::string& output)
   std::map <std::string, int> orphans;
   for (auto& i : filtered)
   {
-    for (auto& att : i)
+    for (auto& att : i.data)
       if (att.first.substr (0, 11) != "annotation_" &&
           context.columns.find (att.first) == context.columns.end ())
         orphans[att.first]++;