[InverseDynamics] Add custom namespaces per configuration

BulletInverseDynamics can be configured to compile using Eigen
instead of Bullet's LinearMath.

Adds a preprocessor defined specific namespace for each
configuration to produce linker errors if incompatible
libraries are linked (eg, builtin core library and
utilities for bullet types).

src/BulletInverseDynamics/MultiBodyTree.hpp

@@ -1,6 +1,7 @@
 #ifndef MULTIBODYTREE_HPP_
 #define MULTIBODYTREE_HPP_
 
+#include "IDConfig.hpp"
 #include "IDMath.hpp"
 
 namespace btInverseDynamics {
@@ -304,5 +305,4 @@ private:
     InitCache* m_init_cache;
 };
 }  // namespace btInverseDynamics
-
 #endif  // MULTIBODYTREE_HPP_