Parallelize LOR loading in mlem

[jacg]

When reconstructing > 800 million LORs with OSEM on a decent number of threads, the time taken to

1. read the LORs from file
2. fill scattergram
3. convert HDF5 LORs into MLEM LORs

takes a significant proportion of the total time.

Step 3 should be trivially parallelizable.

2 would require separate scattergrams to be filled on each thread, and joined together at the end, similar to what is being done with images in the MLEM/OSEM iterations.

As for 1, this involves reading from SSD and I have no idea whether this can be parallelized sensibly.