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
```

まず、現在の multiplicative-partition 実装を特定し、grid_shape のリファクタリング後も非推奨の create_factors ヘルパーが残っているか確認します。Issue で説明されている 2 つのアプローチを比較し、そのうえで、1 の値を含む grid shape も含め、残っているメソッドを独立したテストでカバーできるか判断します。完了の条件は、サポートされているケースに対してそのテストが有用であり、パスすることです。

索引モデルが issue の本文から書いたものです。