glyenzy 0.8.1

Minor improvements and fixes

• autoplot() now accepts the four glydraw 0.8.0 orientation values: "left", "right", "up", and "down".
• Enzyme rules for MGAT4 and MGAT5 have been updated.
• trace_biosynthesis() now infers max_steps from the largest target glycan when it is NULL, using its monosaccharide and substituent count plus 4 for N-glycans and minus 1 otherwise; explicit limits remain supported. (#40)
• trace_biosynthesis() now returns a single-vertex target network when the target is already the selected starting glycan and no enzymatic steps are required. (#39)