Handle Akvo subtypes in descriptions

[rolfkleef]

In merge-activities, we group by description type and add narratives per type. Akvo description subtype attributes may be added (to be verified), but there will be only one subtype attribute.

When multiple Akvo subtypes are included for a description type, this information is currently lost.

Explore whether to add an extra subtype grouping on Akvo attributes.

And if so: verify other elements, and perhaps the Lux maee extension too.