Add GET consumption endpoint

With prod credentials:

In [1]: from pymasmovil.models.session import Session                                                                                                           
In [2]: session = Session.create()                                                                                                                              
In [3]: from pymasmovil.models.asset import Asset                                                                                                               
In [4]: Asset.get_consumption(session, '02i4I00000b9JTOQA2')                                                                                                    
Out[4]: 
{'vozNacionalTotal': 'ILIM',
 'vozNacional': '5',
 'volumenTotal': '5120',
 'volumen': '66.0',
}