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

tests/sandbox/meson.build

@@ -0,0 +1,14 @@
+sandboxes = [
+    'discritization_sandbox',
+    'eos_sandbox',
+    'finite_sandbox',
+]
+
+foreach sandbox: sandboxes
+  executable(
+    sandbox,
+    sandbox + '.cpp',
+    dependencies: [serif_dep, stroid_dep],
+    build_rpath: mfem_runtime_prefix == '' ? '' : mfem_runtime_prefix / 'lib',
+  )
+endforeach