Adapt mixing module for usage on GPUs

[lars2015]

Is your feature request related to a problem?

• The new mixing module in trac.c is limited to operating on CPUs. For GPU runs, particle data will be transferred form GPU to CPU memory, mixing will be calculated on the CPUs, and data will be transferred back to GPUs.

Describe the solution you'd like

• Mixing should be calculated directly on the GPUs, avoiding the data transfers between CPU and GPU memory.

Additional context

• The code might be rewritten for GPUs, following the example of module_sort().

[lars2015]

The function module_mixing() has been ported to and successfully tested on GPUs:

https://github.com/slcs-jsc/mptrac/blob/54e6d58e0fbe632a2c285e0e626e49fbd3c47dc8/src/trac.c#L1913

Best performance was found using #pragma acc atomic update for updating the grid box averages:

https://github.com/slcs-jsc/mptrac/blob/54e6d58e0fbe632a2c285e0e626e49fbd3c47dc8/src/trac.c#L2044