adapt to latest libff, interface renamed -> IBody

Sample/HardParticle/IFormFactorPolyhedron.h

@@ -21,7 +21,7 @@
 
 namespace ff {
 
-class IPolyhedron;
+class IBody;
 
 }
 
@@ -41,7 +41,7 @@ protected:
     void setPolyhedron(const ff::Topology& topology, double z_bottom,
                        const std::vector<R3>& vertices) const;
 
-    mutable std::unique_ptr<ff::IPolyhedron> pimpl;
+    mutable std::unique_ptr<ff::IBody> pimpl;
 
 private:
     Span exec_spanZ(const IRotation* rotation) const override;