Add and test intersectionPointOfTwoSegments() in Math2D
simplify ShapeFunction interface, remove result parameter in values()

  - modify MLS and MLSTest accordingly
  - modify Linear2D accordingly
  - modify WeakformProcessor2D and make it more clean
Add MixResult in WeakformProcessor2D and modify Mixer.mix result type
Modify ModelSearcher interface and improve its implementation and user

  - create class ModelSearchResult
  - accordingly modify Model2D, WeakformProcessor2D EnsureNodesNum and their tests (if exists)
  - pass all tests and python demo of Timoshenko standard cantilever
Remove the third "output" parameter of WeakformProcessor2D.PostProcessor.value()