What's Changed

• Rewrote cumsum to use a different code path from xgcm.apply_as_grid_ufunc internally,
  which makes it less susceptible to subtle bugs like the one reported in #507. (#558).
  @TomNicholas
• Improved error message to suggest rechunking to a single chunk when trying to perform disallowed operations
  along chunked core dims.
  By @TomNicholas
• Fix bug where chunked core dims of only a single chunk triggered errors. (#558, #518, #522)
  By @TomNicholas