change Circle from distance function to distance square function
rename Circle to CircleLevelSet
move CircleTest in mf to tb and rename to CircleLevelSetTest
make SampleFunction of TriangleContourBuilderDemo take the benefits of CircleLevelSet and LogicalMinimum
add Newton's method feature into TriangleContourBuilder
let NewtonSolver automatically setDiffOrder()
improve LogicalHeaviside when exp(-2kx) become infinite
fix a bug of CircleLevelSet order 1 differential
improve the start point of Newton method in TriangleContourBuilder
add a "Use Newton" switch to TriangleContourBuilderDemo