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/include/cppad/local/is_pod.hpp

@@ -0,0 +1,79 @@
+# ifndef CPPAD_LOCAL_IS_POD_HPP
+# define CPPAD_LOCAL_IS_POD_HPP
+/* --------------------------------------------------------------------------
+CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-20 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.
+---------------------------------------------------------------------------- */
+/*
+$begin is_pod$$
+$spell
+    nullptr
+    CppAD
+    namespace
+    bool
+    inline
+$$
+
+$section The is_pod Template Function$$
+
+$head Default Definition$$
+The default template definition is that
+$codei%
+    is_pod<%Type%>()
+%$$
+is false for all types.
+
+$head Fundamental Types$$
+This file specializes $codei%is_pod<%Type%>%$$ to be true where $icode Type$$
+is any of the c++11 fundamental types that hold data; i.e.,
+$code void$$ and $code nullptr_t$$ are excluded.
+
+$head Other Type$$
+You can inform CppAD that a particular $icode Type$$ is plain old data by
+defining
+$codei%
+    namespace CppAD { namespace local {
+        template <> inline bool is_pod<%Type%>(void) { return true; }
+    } }
+%$$
+$end
+*/
+namespace CppAD { namespace local { // BEGIN_CPPAD_LOCAL_NAMESPACE
+    //
+    template <class T> inline bool is_pod(void) { return false; }
+    // bool
+    template <> inline bool is_pod<bool>(void)                   {return true;}
+    // short
+    template <> inline bool is_pod<short int>(void)              {return true;}
+    template <> inline bool is_pod<unsigned short int>(void)     {return true;}
+    // int
+    template <> inline bool is_pod<int>(void)                    {return true;}
+    template <> inline bool is_pod<unsigned int>(void)           {return true;}
+    // long
+    template <> inline bool is_pod<long int>(void)               {return true;}
+    template <> inline bool is_pod<unsigned long int>(void)      {return true;}
+    // long long
+    template <> inline bool is_pod<long long int>(void)          {return true;}
+    template <> inline bool is_pod<unsigned long long int>(void) {return true;}
+    // Character types
+    template <> inline bool is_pod<char>(void)                   {return true;}
+    template <> inline bool is_pod<signed char>(void)            {return true;}
+    template <> inline bool is_pod<unsigned char>(void)          {return true;}
+    template <> inline bool is_pod<wchar_t>(void)                {return true;}
+    template <> inline bool is_pod<char16_t>(void)               {return true;}
+    template <> inline bool is_pod<char32_t>(void)               {return true;}
+    // floating point types
+    template <> inline bool is_pod<float>(void)                  {return true;}
+    template <> inline bool is_pod<double>(void)                 {return true;}
+    template <> inline bool is_pod<long double>(void)            {return true;}
+
+} } // END_CPPAD_LOCAL_NAMESPACE
+
+# endif