Add single-file analysis

[schroedtert]

Currently PedPy can deal with single-file experiment data, but it could be improved. Following additions might be useful:

• project the 2d movement (circle, ellipse) into a 1d movement (line) as pre-processing step
• automatic detection of person in-front and behind
• headway
• density in single-file

An inspiration could be this repo: SingleFileMovementAnalysis