feat(SpectralSolver): Began work on multizone spectral solver

The single zone solver we have is too slow for a true high resolution
multi-zone enviroment. Began work on a spectral element method
multi-zone solver

tests/graphnet_sandbox/meson.build

@@ -3,3 +3,9 @@ executable(
     'main.cpp',
     dependencies: [gridfire_dep, cli11_dep],
 )
+
+executable(
+    'spectral_sandbox',
+    'spectral_main.cpp',
+    dependencies: [gridfire_dep, cli11_dep]
+)