New port: math/adept: Combined array and automatic differentiation library in C++

This library implements a very unusual and interesting approach. It takes a function
of several arguments supplied by the caller, deconstructs its analytic structure
using C++ type substitution for all values, and computes gradients (Jacobians) of
the function while ony calling the function once. A very innovative approach.

Submitted by: myself
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D13635