Add ifdef allowing to use double instead of float.
Add ifdef allowing to use double instead of float.

msteinbeck/tinyspline,msteinbeck/tinyspline,msteinbeck/tinyspline,msteinbeck/tinyspline,msteinbeck/tinyspline,msteinbeck/tinyspline,retuxx/tinyspline,msteinbeck/tinyspline,retuxx/tinyspline,msteinbeck/tinyspline,retuxx/tinyspline