Geometry methods don't accept atoms=[]

[pfebrer]

When Geometry sanitizes atoms=[] it returns an empty numpy array of dtype float64. That causes problems because, even though it doesn't contain any indices, arrays don't accept it as an indexer, as it is not of dtype int or bool.

Reproducable code

import sisl
sisl.geom.graphene().sub(atoms=[])

IndexError                                Traceback (most recent call last)
<ipython-input-20-053c248b033e> in <module>
----> 1 sisl.geom.graphene().sub(atoms=[])

~/webDevelopement/sislGUI/sisl/sisl/geometry.py in sub(self, atoms, cell)
   1701         atoms = self.sc2uc(atoms)
   1702         if cell is None:
-> 1703             return self.__class__(self.xyz[atoms, :],
   1704                                   atoms=self.atoms.sub(atoms), sc=self.sc.copy())
   1705         return self.__class__(self.xyz[atoms, :],

IndexError: arrays used as indices must be of integer (or boolean) type

The same error is raised by:

sisl.geom.graphene().sub(atoms=np.array([], dtype=float))

(which makes more sense, but still I don't know if this should raise an exception)
but not by:

sisl.geom.graphene().sub(atoms=np.array([], dtype=int))

[zerothi]

Thanks!