Return properties to users for certain classes

[khider]

To help users write queries, it might be useful to create a function that tells people what properties are available for that class, regardless of the pre-defined ontologies.

I would say the most important ones are:

• [x] Dataset
• [x] Variable
• [x] Model

[khider]

Done in https://github.com/LinkedEarth/pylipd/commit/3d480e35759e99dd627cdfb76fcdfb77dd9853f0