perf(thread saftey): All Engines are now thread safe

Previously engines were not thread safe, a seperate engine would be
needed for every thread. This is no longer the case. This allows for
much more efficient parallel execution

tests/graphnet_sandbox/meson.build

@@ -4,8 +4,8 @@ executable(
     dependencies: [gridfire_dep, cli11_dep],
 )
 
-executable(
-    'spectral_sandbox',
-    'spectral_main.cpp',
-    dependencies: [gridfire_dep, cli11_dep]
-)
+#executable(
+#    'spectral_sandbox',
+#    'spectral_main.cpp',
+#    dependencies: [gridfire_dep, cli11_dep]
+#)