用c++语言实现三弯矩的插值函数，有三种边界条件，:1：已知两端的一阶导数； 2:两端的二阶导数已知； 默认：自然边界条件。-C++ language with the three moments of interpolation function, there are three boundary conditions: 1: Known both ends of the first derivative　2: the second derivative is known at both ends　default: the natural boundary conditions.