MAINT: coord_dims speedup

This commit offers to speedup the comparison of supplied coordinate with
that on the cube by way of checking if its the same object, then
comparing by way of metadata if object not found.  This in particular
was targetting CubeList merging.
Merge pull request #587 from cpelley/merge_path

Optimise Cube.coord_dims()
Merge pull request #593 from ajdawson/plot-1d-extension

Extend 1D plotting capabilities. Resolves #581.
Add show to quickplot.