test(tests): tests for vacuum region

Test suite has been updated to include a much more robust suite of tools for validating mesh quality and mesh usability (including but not limited to things such as validating that integrating over a sphere provides the correct volume, that the mesh is confirming, and that no elements are inverted)

tests/sandbox/default.toml

@@ -0,0 +1,15 @@
+[main]
+core_steepness = 1.0
+flattening = 0.0
+include_external_domain = true
+inf_bdr_id = 2
+order = 3
+r_core = 1.5
+r_infinity = 10.0
+r_instability = 1e-14
+r_star = 5.0
+refinement_levels = 2
+surface_bdr_id = 1
+core_id = 1
+envelope_id = 2
+vacuum_id = 3