Refactor transformation classes to better support slicing of WCS
Generalize find_coordinate_range to arbitrary number of coordinates
Generalize CoordinatesMap to arbitrary number of coordinates, with slicing
Transformation default is now pixel to world since it is always defined
Add slicing and arbitrary number of coordinates to top-level WCSAxes class
Refactor tick rotation calculation to depend only on pixel-to-world transformation.
Add a new experimental mode for drawing grid lines using contour - this avoids ever needing a world-to-pixel transformation which is ill-defined if distortions are present or when slicing a cube.
Pass slices to reset_wcs
Fix order of transformations following refactoring
Fix bug in tick rotation
Fix ticks for origin='upper'
Make grid_helper an option for grid
Fix plotting for cubes when grid is not requested
Fixes for naxis > 2
Fix failure when 'slices' is not set
Merge pull request #24 from astrofrog/refactor-wcs-handling

Refactor WCS handling