docstr: Initialize dictionary for filter operations.
code: def _init_get_dict():
      """Initialize dictionary for filter operations."""
      return {
          "main chain": PandasMmtf._get_mainchain,
          "hydrogen": PandasMmtf._get_hydrogen,
          "c-alpha": PandasMmtf._get_calpha,
          "carbon": PandasMmtf._get_carbon,
          "heavy": PandasMmtf._get_heavy,
      }
  
imports: from __future__ import annotations
  import os
  import copy
  import gzip
  import warnings
  from string import ascii_uppercase
  from typing import Any, Dict, List, Union
  from warnings import warn
  import numpy
  import pandas
  from looseversion import LooseVersion
  from mmtf import MMTFDecoder, MMTFEncoder, fetch, parse, parse_gzip
  from biopandas.constants import protein_letters_3to1_extended
  from ..pdb.engines import amino3to1dict, pdb_df_columns, pdb_records
label_desc: Use image node_id 9 for calling the PandasMmtf obj's underlying member method code with example usage: obj._init_get_dict() and returns: dict