v0.28.0-rc1

• Multigather is now made the default (and also improved). Some search settings
  have changed meaning, so if you have modified values please discard them.
  Specifically, max-collision-events, max-collision-visits and
  max-out-of-order-evals-factor have changed default values, but other options
  also affect the search. Similarly, check that your gui is not caching the old
  values.
• Performance improvements for the cuda/cudnn backends.
• Support for policy focus during training.
• Larger/stronger 15b default net for all packages except android, blas and dnnl
  that get a new 10b network.
• The distributed binaries come with the mimalloc memory allocator for better
  performance when a large tree has to be destroyed (e.g. after an unexpected
  move).
• The legacy time manager will use more time for the first move after a long
  book line.
• The --preload command line flag will initialize the backend and load the
  network during startup.
• A 'fen' command was added as a UCI extension to print the current position.
• Experimental onednn backend for recent intel cpus and gpus.