Different behaviors of division in python 2 and python 3

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Many files in this project uses some sort of division, but didn't do from __future__ import division. As a result, there are some bugs specific to the python version.

For example, in python 2, Pauli.__str__ will try to use the sparse format for very dense Paulis:

>>> q.Pauli('XXXXXXXXXXI')
i^0 XXXXXXXXXXI
>>> str(_)
'i^0 X[0] X[1] X[2] X[3] X[4] X[5] X[6] X[7] X[8] X[9]'

Because in this line, the division is treated as integer division:

if len(self) <= SPARSE_NQ or self.wt/len(self) > SPARSE_THRESH:

On the other hand, in python 3, generic_clifford does not work at all, because this division will return a floating point number:

nq=len(paulis_in)/2

Causing an error to be raised when using nq as a slice index.

Those problems should be fixed by from __future__ import division in both files, and changing the latter instance of / to //.