FIX: Fix dtype in cartesian

[oyamad]

Current code:

>>> nodes = [[0, 1], [2.3, 4.5]]
>>> qe.cartesian(nodes)
array([[0, 2],
       [0, 4],
       [1, 2],
       [1, 4]])

This PR:

>>> nodes = [[0, 1], [2.3, 4.5]]
>>> qe.cartesian(nodes)
array([[0. , 2.3],
       [0. , 4.5],
       [1. , 2.3],
       [1. , 4.5]])

[coveralls]

Coverage increased (+0.05%) to 95.281% when pulling 1494995 on fix-gridmake into 366c558 on master.