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

benchmarks/SingleZoneSolver/meson.build

@@ -0,0 +1,5 @@
+executable(
+    'gf_bench_single_zone_parallel',
+    'main.cpp',
+    dependencies: [gridfire_dep],
+)