danforthcenter/plantcv: PlantCV v3.0.dev1

PlantCV v3.0.dev1 is an early release in the v3 release cycle. A full v3 will be released at a later date. While this is an early release, it should be considered essentially production-ready. Some features already included in the v3 release: Distance transformation Release of PlantCV through Docker containerization Full and tested Python2/3 and OpenCV2/3 compatibility Global settings and simplified API framework New boundary line tools Simplified region of interest (ROI) tools Color correction/standardization Package namespace reorganization Documentation improvements and bug fixe