Parsing improvements

[xtrojak]

• insert_atomic_to_complex and insert_struct_to_complex do not check if the replaced agent is unspecified (i.e. atomic has '_' feature and structure has no atomics specified) #72
• Use complexes directly in abstract_sequence #89
• Syntactic grammar improvements #93
• Parsing issue for output rules  #26
• Allow bidirectional rules #30
• Regular regulation - enable only regular expressions over rule labels #88
• Check that rule labels used in regulations do actually exist #95
• Zooming in abstract syntax to original BCSL syntax #98
• Allow more robust rate expressions #87
• replication rules #106
• define observables #108
• structure agent can contain only unique atomic agents (w.r.t. names) in its composition #102
• raise error when an undefined complex alias is used in a rule #103

Contribute changes to branch improve-parsing.