Add a section about autodiff

[certik]

It was discussed at the October 2020 Fortran call to add a section to our website about autodiff, similar to https://www.juliadiff.org/, that would list all the libraries and tools for automatic differentiation and we would keep it up to date as we develop such tools more.

[sgeard]

In the recent lockdown I decided to re-implement some code I wrote about 20 years ago to make it more extensible to higher derivatives. Currently it supports up to and including 4th:

my autodiff

It uses operator overloading, function pointers and type inheritance (order n extends order n-1). Built and tested with gfortran 10.1.0