feat(topology): vacuum coordinate and multi block

Two major changes in this version. First stroid now embeds a vacuum coordinate as part of its StroidMesh file (this is a packed set of mfem meshes and GridFunction). This is a logical coordinate from 0 at the stellar surface to 1 at the mesh surface / compactified infinity which can be used by consumers to much more stablly infer position in the vacuum region. Second, there is a new topology backend, multi_block, which has been made the default. See the readme for more information but the basic jist is that multi_block addes 6 transition blocks onto the edge of the core domain. This allows for a much more well conditioned transition from the internal cartesien region to the external spherical region. The mesh conditioning improves by roughly a factor of 1000 for the same refinement level when compared to the legacy topology. The legacy topology is maintained as a option if core_mapping is set to spherified in the config.

configs/conditioned_core.toml

@@ -0,0 +1,21 @@
+[main]
+core_mapping = "multi_block"
+refinement_levels = 2
+order = 3
+include_external_domain = true
+r_core = 0.25
+r_star = 1.0
+r_infinity = 6.0
+flattening = 0.0
+r_instability = 1e-14
+core_steepness = 1.0
+continuity_order = 2
+surface_bdr_id = 1
+inf_bdr_id = 2
+core_id = 1
+envelope_id = 2
+vacuum_id = 3
+
+[main.optimization_methods]
+tmop = false
+smoothstep = true