feat: early draft PR for SLAM

[fabianschenk]

This is a draft PR for SLAM, with all the basic modules.
It includes our new data structures and Yann's camera models.

Open issues:

• API naming
• Code clean-up
• Run script similar to bin/opensfm_run_all
• More SLAM unit tests
• Check fusion with non-local Landmarks (add more comments)
• PyPangolin SLAM visualization

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