Release v0.2.3: Public dataset downloading

Added

• Command to download public datasets (#582)
• Extension autodetection in ByteImage (#595)
• MPII Human Pose Dataset (import-only) (.mat and .json) (#584)
• MARS format (import-only) (#585)

Changed

• smooth_line from datumaro.util.annotation_util - the function is renamed to approximate_line and has updated interface (#592)
• The pycocotools dependency lower bound is raised to 2.0.4 (#449)

Deprecated

• Python 3.6 support

Fixed

• Fails in multimerge when lines are not approximated and when there are no label categories (#592)
• Cannot convert LabelMe dataset, that has no subsets (#600)