Fix qhull crash during matching on small image set

[pierotofy]

Hey there ✋

When trying to reconstruct a small image set (2 or 3 images) and matching_graph_rounds is enabled, the pipeline will fail at the feature matching step:

Traceback (most recent call last):
  File "/code/SuperBuild/install/bin/opensfm/bin/opensfm_main.py", line 25, in <module>
    commands.command_runner(
  File "/code/SuperBuild/install/bin/opensfm/opensfm/commands/command_runner.py", line 37, in command_runner
    command.run(data, args)
  File "/code/SuperBuild/install/bin/opensfm/opensfm/commands/command.py", line 12, in run
    self.run_impl(data, args)
  File "/code/SuperBuild/install/bin/opensfm/opensfm/commands/match_features.py", line 11, in run_impl
    match_features.run_dataset(dataset)
  File "/code/SuperBuild/install/bin/opensfm/opensfm/actions/match_features.py", line 14, in run_dataset
    pairs_matches, preport = matching.match_images(data, {}, images, images)
  File "/code/SuperBuild/install/bin/opensfm/opensfm/matching.py", line 46, in match_images
    pairs, preport = pairs_selection.match_candidates_from_metadata(
  File "/code/SuperBuild/install/bin/opensfm/opensfm/pairs_selection.py", line 630, in match_candidates_from_metadata
    g = match_candidates_by_graph(
  File "/code/SuperBuild/install/bin/opensfm/opensfm/pairs_selection.py", line 253, in match_candidates_by_graph
    triangles = spatial.Delaunay(points).simplices
  File "qhull.pyx", line 1840, in scipy.spatial.qhull.Delaunay.__init__
  File "qhull.pyx", line 356, in scipy.spatial.qhull._Qhull.__init__
scipy.spatial.qhull.QhullError: QH6214 qhull input error: not enough points(2) to construct initial simplex (need 4)

While executing:  | qhull d Qt Qc Q12 Qbb Qz
Options selected for Qhull 2019.1.r 2019/06/21:
  run-id 521442032  delaunay  Qtriangulate  Qcoplanar-keep  Q12-allow-wide
  Qbbound-last  Qz-infinity-point  _pre-merge  _zero-centrum  Qinterior-keep
  _maxoutside  0

This is solved by having match_candidates_by_graph return early if there's less than 4 input images.

[facebook-github-bot]

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