Add time validity information for datasets in the data repository

[ChristianBeilschmidt]

The data repository contains a list of datasets and layers. However, it is not clear when these data are valid. Unfortunately, time range information is not mandatory on these datasets but rather an optional element.

When investigating the request mapping_cgi?service=USER&request=sourcelist, which outputs the datasets for the repository view, one can see that some datasets and layers contain time_start and time_end information, e.g. time_start: "1979-01-01T00:00:00Z".

So when possible, show the time information in the repository view. As a first step, when there is time information on the dataset level, show the time validity beneath the dataset description. Then, add a third column to the list of layers, if there is separate time information for the layers.

As a side note, the parsing of the datasets is done in the user service.