proposal: allow for interdependent parameter value lists of operation parameters

[kaestli]

often the available choices, or valid parameter ranges of one parameter in a parameter value list depends on the choice done on another parameter

(example: depending on which crs [coordinate system] is selected in a wms request, the range of valid coordinate values of the bounding box are different)

epos dcat does not allow to represent this. As a result, in order to avoid invalid parameters, each valid combination of parameters must be represented as individual data set, distribution, web service, and operation. This is a major drawback for the usability of applications built on top of this metadata framework.

epos dcat needs a way to hierarchically represent parameter lists.