STY: usage of itertools

[jklenzing]

Is your feature request related to a problem? Please describe.
There are areas of the code where we have a bunch of nested for loops. This limits the usable space deep in the loop (80-char).

Describe the solution you'd like
Use itertools.product to combine the loops. This is part of the core python package. Functionally, it still works as nested for loops, but the ease of reading is improved.

Describe alternatives you've considered
Leave things as they are.

Additional context
Example:

pysat/pysat/instruments/methods/testing.py

Lines 634 to 636 in 6389d9d

	for version in versions:
	for revision in revisions:
	for cycle in cycles:

import itertools
a = [1, 2]
b = [3, 4]
c = [5, 6]
for i, j, k in itertools.product(a, b, c):
    print(i, j, k)

[aburrell]

This is more pythonic, so I like it. However, I think there should be a comment above that explains it's basically nested for-loops for ease of reading.