Independent test of grid factoring methods.

To address #258, we are making changes to allow 1 values in grid_shape. Previously, we had two methods for computing the factors, I think one slow but simpler, and one fast and more subtle. Only the second one of these is surviving the refactor for this, as the first is effectively unused code. But, the deprecated method being removed might be useful as an independent test of the factorization method that we do use. This deprecated method proved difficult to adapt to allowing the 1 values, which also argued for its removal.

But in case this alternate method is later useful for testing, I note the code used here, so it is not lost track of.

```
def test_both_methods(self):
"""
Do the two methods of computing the multiplicative partitions agree?
"""
for s in [2, 3]:
for n in range(2, 512):
self.assertEqual(utils.mult_partitions(n, s),
utils.create_factors(n, s))

def divisors(n):
i = 2
while i>> mult_partitions(52,3)
[(2, 2, 13)]
>>> mult_partitions(52,2)
[(2, 26), (4, 13)]
"""
return [tuple(flatten(p)) for p in mult_partitions_recurs(n,s)]

def mult_partitions_recurs(n, s, pd=1):
if s == 1:
return [n]
divs = divisors_minmax(n, pd, int(sqrt(n)))
fs = []
for d in divs:
fs.extend([(d,f) for f in mult_partitions_recurs(n/d, s-1, pd)])
pd = d
return fs
```

Inizia individuando l’implementazione attuale di multiplicative-partition e verificando se gli helper create_factors deprecati sono ancora presenti dopo il refactoring di grid_shape. Confronta i due approcci descritti nell’issue, quindi determina se un test indipendente può coprire il metodo rimanente, comprese le forme della griglia contenenti valori 1; il lavoro è completato quando il test è utile e passa per i casi supportati.

Scritto dal modello di indicizzazione a partire dal testo della issue.