docstr: Returns the homology group order of the current cell
docstr_tok: ["Returns","the","homology","group","order","of","the","current","cell"]
code: def homology_group_rank(self):
      """
      Returns the homology group order of the current cell
      """
      if self.hg_n is None:
          self.hg_n = sum(1 for v in self.C if v.minimiser())
  
      return self.hg_n
  
imports: import numpy
  import copy
label_desc: Use image node_id 4 for calling the VertexGroup obj's underlying member method code with example usage: obj.homology_group_rank() and returns: self