update to 1.18
changes:
* Add support for dataset expressions when plotting
* Add axis-function widget for plotting axes which have a scale given
  by a function, or are linked to a different axis via a function
* Add stepped colour maps
* Support editing multiple datasets simultaneously in editor
* Add setting to fix aspect-ratio of graphs
* Add 'vcentre' line step mode for vertical step plots
* Add internal margin setting for grids to separate sub-plots
* Add pixel, pixel_wcs, fraction and linear_wcs FITS import
  coordinate system modes
* Add drop down toolbar button menu to create axis widgets
* More efficient widget dependency resolution
+bugfixes

(drochner)