Add variable group to discovery model

[esloho]

Closes #983

Add the concept variable_group to the discovery model and methods for:

• Getting the variables that belong to a variable group
• Getting the variables_groups for a given dataset

[oleurud]

The code looks cool!

[oleurud]

A question, should we add a variable_group method in Variable?

[esloho]

A question, should we add a variable_group method in Variable?

Current implementation returns the id, as it does with the other properties. However, I think we should consider if that's the behavior we want or if we should return the entity itself and not only the id.

Since we're about to have a new version of the catalog with a different approach, I propose to address this point in the new model and leave this PR as it is.

[oleurud]

Acceptance OK