Require block size be multiple of 8 for cfd

- Without this, we either have to rewrite cfd to not use structs of
  arrays in Juno or deal with getting the alignment of those arrays
  right in Rust, both are annoying

juno_samples/rodinia/cfd/src/main.rs

@@ -146,6 +146,8 @@ fn cfd_harness(args: CFDInputs) {
         pre_euler,
     } = args;
 
+    assert!(block_size % 8 == 0, "Hercules expects all arrays to be 32-byte aligned, cfd uses structs of arrays that are annoying to deal with if the block_size is not a multiple of 8");
+
     let FarFieldConditions {
         ff_variable,
         ff_fc_momentum_x,