Increment ray idx by 1 to avoid zero values in RNG
In the RNG function for rays, increment ray idx by 1 to avoid zero values (ie bits with all zeroes)
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This is the same change as in https://gitlab.cosma.dur.ac.uk/EAGLE/swift-colibre/-/merge_requests/183 (so that swiftsim and swift-colibre are consistent)
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I also added descriptions to two functions whose descriptions were missing for some reason.