This header file can only be used in CUDA source files.

Utility CUDA functions now throw a runtime error wrapping the CUDA
error message when a primitive CUDA function returns an error code.

The LB GPU checking function gpu_init_particle_comm() was rewritten
to check the currently selected GPU instead of the one with ID 0.
The warning about insufficient compute capability was converted
to an error message. The warning about default GPU ID was removed.
The Python-specific message was removed. The function now calls
std::abort() upon any error since we cannot recover from it in
the Python interface (the list of actors cannot be cleared due to
a mismatch between `Actor` and `Actors` regarding active actors).

The dipolar Barnes-Hut code now exits early upon any CUDA error to
avoid undefined behavior in memory allocation on the GPU.

Also fix the type of the memory field to avoid integer overflow.

Convert regular comments to doxygen comments where appropriate,
document function arguments, remove duplicate doxygen blocks.

`import espressomd` shouldn't fail on a machine without GPU when
CUDA is compiled in.