Resolving KNN hang issue for SIFT features.

After the changes for the AKAZE features the SIFT feature matching no longer worked for me for the Berlin data set. The KNN search in the features.match_lowe function hanged indefinitely. I narrowed it down to the [ced8152](https://github.com/mapillary/OpenSfM/commit/ced8152a4c1a65ad6f67be36ab989dbf81397a86) commit and changed the code to avoid converting values that are already float32 to float32 in connection to the FLANN index calls. After that the SIFT feature matching works for me again.