Label setting

Support showing label text for each atom. The label can be any attribute of batoms. Support both Edit and Object modes.

Take CH₄ molecule as a example.

>>> from ase.build import molecule
>>> from batoms import Batoms
>>> import numpy as np
>>> ch4 = Batoms('ch4', from_ase = molecule("CH4"))
>>> ch4.show_label = 'index'
>>> ch4.show_label = 'species'
>>> ch4.show_label = 'elements'
>>> ch4.set_attributes({"charges": np.zeros(5)})
>>> ch4.show_label = 'charges'

None	Index	Species	Charge