Intermediate/long term GPU TODO

[dcmvdbekerom]

This is a list of GPU TODO's that didn't make it into #604;

• move from ctypes to cuda-python backend -> cuda-python has toolkit dependencies, leave it as it is..
• move from cuFFT to VkFFT (with CUDA backend)
• add spectrum.update_gpu() method
• improve cpu-cuda by multithreading and simd -> Tried both, no significant speedup..
• Fix emissivity spectra (currently don't work - likely related to s.units['waverange'] and s.conditions['waveunit'] are duplicate #621)
• non-air diluents
• truncation & overlap add
• non-eq spectra (needs rework on RADIS engine first, making df's rely on A instead of S) #619
• sparse (not sure this is feasible on GPU)

The following old open issues are related to GPU:
#461
#269
#144
#124