added gradient verification

[jhueckelheim]

Added a checksum for the Enzyme gradients of LULESH.jl, and another checksum that is computed using finite differences.
The FD checksum is computed if the option --verification is given, the AD checksum is computed if both options --verification --enzyme are given.

For example:

./mpiexecjl --project -np 1  julia --project examples/benchmark.jl -s 4  --mpi --enzyme --verification

will print gradient checksum: 0.833891, whereas

./mpiexecjl --project -np 1  julia --project examples/benchmark.jl -s 4  --mpi --verification

will print FD checksum: 0.833892 on my machine (small differences are expected due to truncation and roundoff errors).