A more uniform access to bullet world items and robot links

[mpomarlan]

Use cases:

-- I want to associate a bullet object of some sort (depending on what is available, an item, rigid-body, or object) to a designator that is either (an object (name ?name)) or (an object (part-of ?robot) (urdf-name ?urdf-name))
-- I want to see what items are supported by "something", which can be a tray (an item) or the table (a link of the kitchen)

What I needed to do currently to achieve this is to implement my own name-to-object function with appropriate branches for the situation, and similarly in my (not yet merged into CRAM) list-supported-objects code there needs to be a branch that uses either the (supported-by ?t ?b) or (supported-by ?t ?b ?l) predicates.

Particularly annoying is the fact that rigid bodies that are links of a robot have no parent property to query. Instead, one needs to rely on the naming convention to extract the robot name part.

Suggestions:

-- at the very least, rigid bodies should have a parent property.
-- it would be better if robot links were implemented as items AND
-- for the (btr:object world name), name can be either a symbol/keyword or a list of symbols/keywords AND
-- (btr:objects world) returns only the objects for which name is a symbol/keyword; this way only the "top" level objects are listed by it, but nonetheless links are also easily accessible via btr:object when needed.