openbim-step 0.5.1

Added

• SchemaGraph::direct_subtypes and SchemaGraph::subtypes: the downward
  counterpart of supertypes. subtypes(x) is every entity y != x for
  which is_a(y, x) holds, at any depth, in a deterministic sorted
  pre-order. A test checks that equivalence for every ordered entity pair.
  Answering "every IfcElement" needs this; EntityDef records only the
  upward edge, so the child index is built once at construction.