mmcif writing: refactor

- Introduce code redundancy by removing star_base.hh again.
  It was a bad idea from the very beginning. The reader operated
  with ost::StringRef. A StringRef is purely pointer based  which is fine
  when you have external data ownership. Thats the case for reading.
  For writing you want to build up a data structure that actually holds data.
  So you want to store actual strings. Thus the introduction of StarWriter
  objects.
- Use shared_ptr instead of raw pointers
- Change function names to better follow the OST conventions