feat(mean_field): added initial implementation

note this implementation lacks many tests

libmeanfield/interface/physics/context.cppm

@@ -0,0 +1,28 @@
+module;
+#include <mfem.hpp>
+
+export module mean_field:physics.contexts;
+export import :mapping.coefficients;
+
+export namespace mean_field::physics {
+    struct GravityContext {
+        std::unique_ptr<mfem::ParBilinearForm> m_form;
+        std::unique_ptr<mfem::ParMixedBilinearForm> b_form;
+
+        std::unique_ptr<mfem::BlockOperator> block_A;
+
+        std::unique_ptr<mfem::Solver> prec_M;
+        std::unique_ptr<mfem::HypreBoomerAMG> prec_Phi;
+        std::unique_ptr<mfem::BlockDiagonalPreconditioner> block_prec;
+
+        std::unique_ptr<mfem::MINRESSolver> minres;
+
+        mfem::Array<int> stellar_mask;
+
+        std::unique_ptr<mfem::TransposeOperator> BT;
+        std::unique_ptr<mfem::HypreParMatrix> Schur;
+
+        std::unique_ptr<mfem::MatrixCoefficient> mapped_hdiv_mass_coeff;
+
+    };
+}