Update get number of atoms function

In Ase 3.22, get_number_of_atoms() is deprecated.
It is suggested, "if your atoms are distributed, use (and see) get_global_number_of_atoms()."

catlearn/optimize/mlmin.py

@@ -73,7 +73,7 @@ def __init__(self, x0, prev_calculations=None, restart=False,
  assert self.ase_calc, msg
  self.constraints = self.ase_ini.constraints
  self.x0 = self.ase_ini.get_positions().flatten()
- self.num_atoms = self.ase_ini.get_number_of_atoms()
+ self.num_atoms = self.ase_ini.get_global_number_of_atoms()
 
  # Information from the initial structure.
  new_atoms = self.ase_ini