Resolving KNN hang issue for SIFT features.

When converting the float32 features to new float32 values the SIFT feature matching hanged in the KNN search in the features.match_lowe method.
Now the float32 conversion when building and loading the FLANN index is only done when the type of the feature vector values are not float32.
Merge pull request #6 from oscarlorentzon/bug/symmetric-matching-hangs

Resolving KNN hang issue for SIFT features.