feat(mean_field): added dimensions, discritization, and start of eos

The full rewrite of mean_field into something maintainable is progressing. dimensions is mostly done, discritization (domain, blocks, and fields) is done, and eos is progressing quickly

src/include/serif/discretization/domain/ids/boundary.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <string_view>
+
+#include "serif/discretization/domain/concepts.hpp"
+
+// BoundaryID binds a semantic surface (a Boundary) to the integer boundary
+// attribute MFEM stores in the mesh file.
+//
+//     BoundaryID<StellarSurfaceBoundary, 1>
+//         "boundary elements with attribute 1 are the stellar surface"
+namespace serif::discretization::domain::ids {
+    template <IsBoundary BoundaryT, int IdentifierValue>
+    struct BoundaryID {
+        using boundary_type = BoundaryT;
+        static constexpr int ID = IdentifierValue;
+    };
+
+    struct BoundaryDescriptor {
+        std::string_view name;
+        int ID;
+    };
+}