Fix warn_unused_result in hyperbolic_upwind.cpp

There is a warning in the compile for hyperbolic1D_upwind.cpp:
```
mole/examples/cpp/hyperbolic1D_upwind.cpp:127:11: warning: ignoring return value of ‘int system(const char*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  127 |     system("gnuplot -persistent gp_script");
```
which should be rectified.