mmcif writer: read chain type from ost.mol.ChainHandle.GetType property

There are two modes controlled by the mmcif_conform flag

- if false: split chains into valid mmcif chains where we do some best guesses
  if _entity.type and _entity_poly.type
- if true: Assume valid mmcif chains and determine _entity.type and _entity_poly.type
  from ost.mol.ChainHandle.GetType