Merge remote-tracking branch 'upstream/v1.5.x'
Merge remote branch 'upstream/v1.5.x'

* upstream/v1.5.x:
  Added example to iris.analysis.interpolate.linear docstring
  Bugfix to DimCoord.from_regular
  Change zorder of contour lines drawn by contourf.
  Modified cube dim_coords and aux_coords docstrings
  Simplified ionosphere example. Closes #603.
  Use space_weather.nc instead of missing file and enabled doctests
Ensure consistent memory layout for checksums.