add toString()
make arg of connect* less restrict (null is ok now)
add Fissionizer, a helper tool
add TriangleMarching contour builder
clear old models
add MFFacet
update cell package to geom
add more utils to model.geom package

  + prepare for migrate tb project's solid element to MF*
migrate to MF* geom elements

  + Segment, Line -> MFLine
  + Segment2DUtils -> MFLine2DUtils
  + GeomUnit -> MFGeomUnit
  + Facet -> MFFacet
  + Chain -> Chain.head -> MFLine
  + remove EnsureNodesNumTest