fixed volatile variables in cuda kernel

hysop/backend/device/codegen/functions/custom_atomics.py

...ce/codegen/symbolic/functions/custom_symbolic_function.py

hysop/backend/device/opencl/opencl_elementwise.py

hysop/numerics/fft/fft.py

hysop/numerics/fft/opencl_fft.py

hysop/operator/base/custom_symbolic_operator.py

hysop/operator/base/spectral_operator.py

hysop/symbolic/misc.py