Better support for categorical parameters

[letmaik]

When trying to view the example in opengeospatial/CoverageJSON#131 in the playground, the categorical vertical profile is rendered as line plot. It should be using some other visualization to indicate the category instead, like it does for grids with a legend.

[KoalaGeo]

For the borehole example, the data would suit https://code.agilescientific.com/welly/userguide/Wells.html for downhole numeric data & https://code.agilescientific.com/striplog/ for the categorical data if there's any code which could be reused from those projects or alternatively adding covjson support to them....