Refactoring: Moved viennagrid::meta::null_type -> viennagrid::null_type

Reduces compiler message length if something goes wrong...
Refactoring: Merged viennagrid::meta::typelist and viennagrid::meta::typemap namespaces

Makes types shorter and easier to find.
Moved viennagrid::meta::typelist_t -> viennagrid::typelist.

Results in considerably shorter types and compiler error messages.
Moved viennagrid::meta::{out_of_range,not_found,static_pair} to viennagrid:: namespace.

Makes a couple of types (and thus compiler messages) considerably shorter.