OEZap::GetFocusTarget now return a pointer instead
of a reference. This is a breaking change and any code that calls
these methods will have to be updated. These methods will return a
null pointer if an acceptable molecule has not been set for them.
A warning has been added for molecules passed into Zap TK that do
not return 3 when GetDimension is called. The dimension
must be 3 for all molecules passed into Zap TK. The dimension is
set automatically when using OEReadMolecule but a warning will
be thrown and the molecule will not be accepted by Zap TK if the
molecule has been made from scratch in the toolkits and
SetDimension(3) has not been called on it.