docstr: Return the xheight as float.
code: def get_xheight(self):
      """Return the xheight as float."""
      return self._header[b"XHeight"]
  
imports: from collections import namedtuple
  import logging
  import re
  from ._mathtext_data import uni2type1
label_desc: Use image node_id 20 for calling the AFM obj's underlying member method code with example usage: obj.get_xheight() and returns: self