hocr 0.0.0.9002

Added bbox_slice functions and tests
Spin off bbox_union2 for pairs of bboxes
Introduce flexible aggregation functions for bbox_union
Added separate_bbox_cols function

hocr 0.0.0.9001

Ditched dependency on XML, added dependency on xml2.
Added basic test to check dimensions of the exported data.

hocr 0.0.0.9000

Initial package release.
Added a NEWS.md file to track changes to the package.