Added RootSIFT extractor
Clean up abstract feature extractor

get_features shouldn’t be static
don’t need properties
add local feature encoders
move extractors
add SIFT => RootSIFT conversion function
Add image encoders

NOTE:

- IMAGE ENCODERS map from images to image descriptors

- FEATURE ENCODERS map from local features to an image descriptor

- on some cases image encoders will use feature encoders
add precomputed histogram_encoder, delete codebook
start working on pipeline