Fix #925: inconsistent return in df_integrals.py::double_factorize (#1111)

The `double_factorize` method in the `DFABKpointIntegrals` class had an
inconsistent return. It was type-hinted to return `None`, but had a code
path that returned a value. This early return also caused a caching
issue where the method would incorrectly return a cached result even
when called with a new `thresh` value, preventing re-computation.

This commit removes the premature return statement. This allows the
`thresh` parameter to take effect as intended and makes the method's
behavior consistent with its type hint.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: mhucka

src/openfermion/resource_estimates/pbc/df/df_integrals.py

@@ -86,7 +86,6 @@ def __init__(self, cholesky_factor: npt.NDArray, kmf: scf.HF):
                 self.reverse_k_transfer_map[kidx, kmq_idx] = qidx
 
         # set up for later when we construct DF
-        self.df_factors = None
         self.a_mats = None
         self.b_mats = None
 
@@ -168,8 +167,6 @@ def double_factorize(self, thresh=None) -> None:
         """
         if thresh is None:
             thresh = 1.0e-13
-        if self.df_factors is not None:
-            return self.df_factors
 
         nkpts = self.nk
         nmo = self.nao