Add c'tors to Compound class

https://bornagainproject.org/git-main/ref/sample/particle/composition/compound/ describes

# creates an empty particle composition
composition = Compound()

# creates a composition made of a single particle at a given position
composition = Compound(particle, position = R3(0,0,0))

# creates a composition and includes copies of a particle at specified positions
Compound(particle, positions = [pos1, pos2, ...])

But currently there is only Compound() constructor.

Should we add the others?

changed milestone to %v22

added discuss in team! label

assigned to @m.svechnikov

Rather, remove unimplemented feature from the docs.

mentioned in merge request !2959 (merged)

mentioned in commit e5dcde00

closed with merge request !2959 (merged)