Fix an integer division on Python 3

With Python 2, '/' returns the floor of the mathematical result of
division if the arguments are ints. With Python 3, the operator
returns a reasonable approximation of the mathematical result of the
division.

This commit uses '//', which is the floor division operator in both
Python 2 and Python 3, so that the result can be safely used to index
an array.

For more details, see PEP 238 "changing the division operator"

  http://www.python.org/dev/peps/pep-0238

Signed-off-by: Christophe Vu-Brugier <cvubrugier@yahoo.fr>