STEPnode::_null had no default: STEPnode has no constructor, so a node built
by a subclass that does not assign it starts out with whatever was on the
heap, and is_null() then answers at random. Give STEPnode a constructor that
starts nodes null.
EntityNode::StrToVal() and ::STEPread() additionally have to set it per
outcome - they resolve an entity reference and must report null exactly when
the reference did not resolve. Before this, a node whose reference failed to
resolve could still report itself non-null.
We want to be consistent with the previous layout, to avoid needlessly
breaking existing codes. The header update means client codes are not
required to include all of the stepcode library includes explicitly -
i.e., stepcode itself does not force this requirement with its own
internal includes. However, if client codes do not want to add the
library prefixes to their own includes as we have for stepcode, they
should still be able to add the various includes and work as before.
Adjust the include directory names accordingly.
Attila Prokai