build(CppAD): brought in CppAD for autodiff

we need an autodiff library at some point (or we need to roll our own but I do not think that makes sense). CppAD is well tested and header only and easy to include. It is also Liscene compatible with GPL v3.0. Here we bring it in as a dependency

build-config/cppad/readme.md

@@ -0,0 +1,50 @@
+# Title
+CppAD: A Package for Differentiation of C++ Algorithms
+
+# Links
+
+- [docmentation](https://coin-or.github.io/CppAD/doc)
+
+- [News](https://coin-or.github.io/CppAD/doc/whats_new.htm)
+
+- [Install](https://coin-or.github.io/CppAD/doc/install.htm)
+
+- [Directories](https://coin-or.github.io/CppAD/doc/directory.htm)
+
+- [Coin-OR Download](https://www.coin-or.org/download/source/CppAD/)
+
+
+# License
+<pre>
+CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-18 Bradley M. Bell
+
+CppAD is distributed under the terms of the
+             Eclipse Public License Version 2.0.
+
+This Source Code may also be made available under the following
+Secondary License when the conditions for such availability set forth
+in the Eclipse Public License, Version 2.0 are satisfied:
+      GNU General Public License, Version 2.0 or later.
+</pre>
+
+
+# Autotools
+The preferred method to test and install CppAD uses cmake.
+The deprecated autotools procedure can be used for this purpose,
+but it will eventually be removed.
+For any sub-directory *dir*,
+files of the form *dir*/`makefile.am` and *dir*/`makefile.in`
+are used to support the autotools test and install procedure.
+In addition,
+the following files, in this directory, are also for this purpose:
+`compile`,
+`config.guess`,
+`config.sub`,
+`configure.ac`,
+`depcomp`,
+`install-sh`,
+`missing`.
+
+
+# Copyright:
+See the file `authors` in this directory.