Columns

- Refactored column objects to contain a ::validate method, for the
  validation of incoming data.
- Context.columns is now a vector of one of each column object,
  indexed by attribute name, for validation purposes.

src/Context.cpp

@@ -113,12 +113,17 @@ void Context::initialize (int argc, const char** argv)
   // Instantiate built-in command objects.
   Command::factory (commands);
 
+  // Instantiate built-in column objects.
+  Column::factory (columns);
+
   // Finally categorize all arguments.
   args.categorize ();
 
   // TODO Instantiate extension command objects.
   // TODO Instantiate default command object.
 
+  // TODO Instantiate extension column objects.
+
   // TODO Instantiate extension UDA objects.
   // TODO Instantiate extension format objects.