Add model.implicit package and add TriangleContourCell
add a TriangleContourBuilder
make a TriangleContourCellFactory and build a demo of TriangleContourBuilder
add Segment2D.link() utility
Update TriangleContourBuilder and TriangleContourCell to adapt unclosed contours

  - Add TriangleContourCell.getContourDesinationEdge()
  - let border TriangleContourCell.netContourCell() be able to return null
  - Add TriangleContourBuilder.tryMergeWithOpenRingHeads()
  - clean TriangleContourBuilder codes
let TriangleContourBuilderDemo react when dragging radius of both inner hole and outer solid