feat(mean_field): added initial implementation

note this implementation lacks many tests

libmeanfield/interface/physics/solid.cppm

@@ -0,0 +1,9 @@
+module;
+#include <mfem.hpp>
+
+export module mean_field:physics.solid_body;
+export import :fem;
+
+export namespace mean_field::physics {
+    double compute_moment_of_inertia(const fem::FEM &fem, const mfem::GridFunction &rho_ref);
+}