generalize TriangleCell to WingedCell
move WingedEdge and its direct children to tb.solid.triangle
rename tb.solid.triangle to tb.solid.winged
restrict WingedEdge CELL parameter that only extends WingedCell
add WingedCellUtils
add TriangleContourCellEdge
let TriangleContourCell implements TriangleCell instead of AdaptiveTriangleCell