Make attach_indices work for both primitives and predicates

[aprokop]

Right now, attach_indices only works for primitives. For predicates, there is no convenient way to attach indices to predicates. The attach only works on a single predicate and not limited to indices.

Attaching indices to primitives and predicates result in different types and access methods. Primitives result in PairValueIndex<Value, Index>, while predicates result in an unspecified type that the index can be accessed by getData().

I'm not sure how how much of a problem returning different types is. The function serves the same role for both primitives and predicates: conveniently adjust passed in data in some easy way for the hierarchy. Both types should really be undefined. The difference is that in the first case the argument appears in the hierarchy template specialization.

The two types should be different, though. The predicate attachment inherits from the predicate and may provide distance() function; the access to the data is also done on the user side, thus getData() function. The primitive attachment is a simple storage and will only be used internally.

[dalg24]

Please do provide a description.

Discuss whether you think that it is a problem that

• when attaching indices to primitives, one get PairValueIndex and access the index via the named data member, while
• when attaching indices to predicates, one get an unspecified type and access the index via getData

[aprokop]

@dalg24 Do you still have concerns about this PR, or can we proceed as is?