Use `emptyset` rather than `source and sink`

[fbergmann]

in SBGN PD the 'source and sink' element was renamed to 'emptyset'. This change is not propagated to SBGNML and libSBGN. Instead the existing 'source and sink' needs to be deprecated and a new 'emptyset' element added to GlyphClazz

[hasanbalci]

It might be better to use empty set instead of emptyset to comply with the naming convention, since it is referred to as 'empty set' in the SBGN PD spec.