Allow (x,y,z) magnetization for particles in any layer (#765)

Allows setting z-magnetization of particles (can be computed without material averaging) in any layer or half-space.

Resolves #765 (closed)

Resample/Slice/Slice.cpp

@@ -84,7 +84,7 @@ void Slice::initBField(R3 h_field, double h_z)
 {
     // Temporary forbid z-magnetization in all layers (see issue #654)
     if (m_material.magnetization().z() != 0)
-        throw std::runtime_error("Processing z-magnetization is not implemented yet.");
+        throw std::runtime_error("Processing z-magnetization of layers is not implemented.");
 
     static constexpr double Magnetic_Permeability = 4e-7 * pi;
     m_B_field = Magnetic_Permeability * (h_field + m_material.magnetization());