Grammar improvements

[xtrojak]

This PR covers several smaller issues found by users.

• allowed usage of complex aliases in complexes and nesting complex aliases in their definition (close #33)
• allowed usage of bidirectional rules (#30) and assigned them implicit labels (close #27)
• allowed empty initial state (close #29)
• fixed BCSLeditor output file format - .bcs instead .txt (close #31)

[xtrojak]

Artificial model where all the new features are present

#! rules
r1_T ~ A(T{i}).B()::cell => A(T{a}).B()::cell
r1_S ~ A(S{i}).another_one::cell <=> A(S{a}).another_one::cell 
r2 ~ A()::cell => A()::out 
r1_S2 ~ S{i}:A():another_one::cell => S{a}:A():another_one::cell

#! complexes
random = A().B()
another_one = random.C()