improved sparse formats; fixed one or two bugs on the way and added a faster...

improved sparse formats; fixed one or two bugs on the way and added a faster test with randonm.poisson data

Hi @paleo , as discussed, I try (managed?) to create a merge request with the changes I mentioned. The idea is to have a smoother interface that I will need for reading/saving data.

sparse_space = space.dense_to_space(frames, mask=mask)
sparse_space_few_frames = sparse_space[:100]
sparse_time = sparse_space.to_time_opencl().to_numpy()


# the lines below work with both time and space compacted data
frame = sparse_data.get_frame(4)
t,i=sparse_data.get_time_trace(pixel, return_zeros=False)