use new fcts ff::Prism::radius and volume

Sample/HardParticle/IFormFactorPrism.cpp

@@ -51,13 +51,13 @@ double IFormFactorPrism::topZ(const IRotation* rotation) const
 double IFormFactorPrism::volume() const
 {
     ASSERT(m_validated);
-    return height() * pimpl->area();
+    return pimpl->volume();
 }
 
 double IFormFactorPrism::radialExtension() const
 {
     ASSERT(m_validated);
-    return std::sqrt(pimpl->area());
+    return pimpl->radius();
 }
 
 //! Returns the form factor F(q) of this polyhedron, respecting the offset height/2.