more precise error message on bad "secondary_axis" argument
remove unused method
added an extra Node/Expr stage when parsing. Those Nodes have all the special methods defined, while the good old Expr does not need them anymore. The parsing stage uses a parsing context which includes all the symbols of all entitiies. Its API is {'ent_name': {symbol_name: symbol_instance}, ..., '__entity__': current_entity_name}. A trivial class dedicated to this would probably help make this less verbose...

 This removes the need for the messy conditional context and should pave the way for all sorts of optimizations. TODO: It is kinda ugly for CompoundExpressions. Maybe I should move their implementation to the "Node stage" instead of the "Expr stage".