Add copyright header to doc.go
Keep original comment style (e.g. line comments and block comments).
Fix doc comments containing multiple line comments.
* diff/fd: implement Hessian finite difference, and code cleanups.
This commit primarily adds the Hessian function for finding a finite difference approximation to the Hessian. At the same time, it combines duplicated functionality across the difference routines so that the preludes to all the difference routines look similar
