docstr: Return only heavy atom entries from a DataFrame
code: def _get_heavy(df, invert):
      """Return only heavy atom entries from a DataFrame"""
      if invert:
          return df[df["element_symbol"] == "H"]
      else:
          return df[df["element_symbol"] != "H"]
  
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 12 for calling the PandasMmtf obj's underlying member method code with example usage: obj._get_heavy(df, invert) and returns: df, df