parallel_strereo and tile-size

[oleg-alexandrov]

parallel_stereo takes the option --tile-size which is not documented either for it or for stereo and it does not split the image into tiles of this size. I think this option is inherited from some lower-level base class and this appears confusing. It also takes the option --corr-tile-size which affects tiling when MGM is used but this is also not documented. Lastly, normally we use --job-size-h and --job-size-w to control tiling. All this needs to be sorted out in the doc.