+ Improve performance of calcOpticalFlowSingleScale method
+ Small refactoring

Current results:

IMAGE NAMES          RMSE
Beanbags
Dimetrodon           0.329428
DogDance
Grove2               0.550852
Grove3               1.464699
Hydrangea            0.523277
MiniCooper
RubberWhale          0.367246
Urban2               2.717003
Urban3               4.185070
Venus                0.775422
Walking

Time (for Urban3):
17.490248 sec