add functions to get stored release dates

[HomoCodens]

In timeseriesdb related support I had to do until now by far the most performed action was checking the release date of certain series.

We should add functions to get the latest release date for:

• single ts
• datasets
• collections

[HomoCodens]

also some other "diagnostic" information would be helpful too:

• start
• end
• first/last value
• frequency
• created at
• created by
• etc.

[omuelle]

Those diagnostic infos would be helpful.