TODO added
core: Add stubs for most important class bindings

Some TODOs added.
accessor.hpp: Add macro for single entity getters

Macro creates a getter that returns an boost::optional instead of
an entity. Is used in order to get None in python instead of an
uninitialized entity.
entity.hpp: functions exposin EntityWithMetadata

- Define a function that can expose specialisation of
  EntityWithMetadata<T>
- Let block inherit from EntityWithMetadata<IBlock>