generalize test file handling
equality testing: special case for unicode strings
CameraModel distort(): variable names match undistort()
MultiCameraSystem: add get_camera() method
bugfix: undistort() when rectification is not null
testing: add camera with skewed pixels
CameraModel: eliminate test for skewed pixels in constructor

It's "merely" a bug that PyMVG cannot handle skewed and distorted
pixels and thus we shouldn't prevent such values from being passed in.
testing: skip test that should fail
CameraModel: specify assumption required for correct math
CameraModel: remove intermediate names
CameraModel: allow skew when projecting pixel to 3D
CameraModel: mirroring of skewed cameras is not implemented (yet)
testing: skip test when NotImplementedError comes up
encode name as ASCII
Merge branch 'skew-fix'. Closes GH-1.