嵌入式算法：STM32 F4xx 使用DSP实现cubic spline插值算法

参考官方文档：

Linear Interpolate Example

Cubic Spline Interpolation

cubic spline的基本概念：

定义：Spline interpolation is a method of interpolation where the interpolant is a piecewise-defined polynomial called "spline"

计算过程：A is a tridiagonal matrix (a band matrix of bandwidth 3) of size N=n+1. The factorization algorithms (A=LU) can be simplified considerably because a large number of zeros appear in regular patterns. The Crout method has been used: 1) Solve LZ=B

计算已知点两端范围外的点：It is possible to compute the interpolated vector for x values outside the input range (xq<x(1); xq>x(n)). The coefficients used to compute the y values for xq<x(1