todo

该文件为c++的数学函数库!是一个非常有用的编程工具.它含有各种数学函数,为科学计算、工程应用等程序编写提供方便！

1. need to add a test for the akima splines

2. [DOCUMENTATION] From: Conrad Curry <[e-mail]>

  I would suggest adding more about cspline and Akima similiar to what
  is given for 'polynomial', particularly under what conditions one
  would be prefered over the other.

3. add akima splines without the Wodicka modification, so that the spline
is a smooth curve without corners.