Making listing the filtering terms vs using filtering terms more homogeneous

[costero-e]

Second point of the solution proposed by @mbaudis in #93
In case we are adding an array of the possible scopes for every filtering terms, then we could discuss if we need this, but I guess it would be practical (and necessary) to extend a filteringTerm endpoint for each scope so we can fetch this information by scope. Having to look always at each scope for each object in a "unique" filteringTerm endpoint would become very tedious in my opinion.
If in some models these endpoints can't be filled, then they could go for the generic one and it would still work for every type of beacon.

[costero-e]

Closing issue. See #94