Initial implementation of rocktype plotting in mulgrid layer_plot()
Added allrocks parameter to layer_plot(), to allow colourbar showing only rocks used on the layer
Added rockgroup parameter to layer_plot(), to allow grouping of rocktypes via filters, and added new setup_rocktype_plot() method to handle rocktype plots for layer and slice plots
Added ability to plot rock types in mulgrid slice_plot()
Bugfix in mulgrid slice_plot_wells(): was failing if slice line was specified as 'x' or 'y'
Added documentation for new rock type plotting in mulgrid layer_plot() and slice_plot()
Changed so that wellname_bottom parameter for mulgrid slice_plot() defaults to False