Added SIFT features using PyCOLMAP.

[mihaidusmanu]

I implemented a simple interface for VLFeat SIFT from PyCOLMAP.

There might some issue with top keypoints selection - I opened an issue on the PyCOLMAP repository to follow up colmap/pycolmap#8.

I get the following results on South-Building without image downsampling and max_keypoints=10000 (to avoid GPU memory issues during matching):

{
  'mean_reproj_error': 0.450604,
  'mean_track_length': 7.001359,
  'num_input_images': 128,
  'num_observations': 535786,
  'num_observations_per_image': 4185.828125,
  'num_reg_images': 128,
  'num_sparse_points': 76526
}

which seems reasonable 😄.

[sarlinpe]

Merged in #72.