class_docstr: Convenience class to encapsulate tractogram file format.
class_docstr_tok: ["Convenience","class","to","encapsulate","tractogram","file","format","."]
docstr: voxmm -> rasmm affine.
docstr_tok: ["voxmm","-",">","rasmm","affine","."]
code: def affine(self):
      """voxmm -> rasmm affine."""
      return self.header.get(Field.VOXEL_TO_RASMM)
  
imports: from abc import ABC, abstractmethod
  from .header import Field
label_desc: Use image node_id 5 for calling the TractogramFile obj's underlying member method code with example usage: obj.affine() and returns: self