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/wno_conversion.hpp

@@ -0,0 +1,45 @@
+# ifndef CPPAD_WNO_CONVERSION_HPP
+# define CPPAD_WNO_CONVERSION_HPP
+/* --------------------------------------------------------------------------
+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.
+---------------------------------------------------------------------------- */
+
+/*
+$begin wno_conversion$$
+$spell
+    cppad
+    wno
+    cpp
+    hpp
+$$
+
+$section Suppress Suspect Implicit Conversion Warnings$$
+
+$head Syntax$$
+$codei%# include <cppad/wno_conversion.hpp>%$$
+
+$head Purpose$$
+In many cases it is good to have warnings for implicit conversions
+that may loose range or precision.
+The include command above, before any other includes, suppresses
+these warning for a particular compilation unit (which usually corresponds
+to a $icode%*%.cpp%$$ file).
+
+$end
+*/
+
+# include <cppad/configure.hpp>
+# if CPPAD_COMPILER_HAS_CONVERSION_WARN
+# pragma GCC diagnostic ignored "-Wfloat-conversion"
+# pragma GCC diagnostic ignored "-Wconversion"
+# endif
+
+# endif