Add GR backend

[dpo]

GR has a stairs function but it does not appear in the documentation: https://github.com/jheinen/GR.jl/blob/e787dadb261206d4d93b7f6478fa55bb0de6606e/src/jlgr.jl#L1816

GR comes with much fewer dependencies than Plots.jl and is much faster.