Expand operators before writing JSON

When a Decapode has composed operators, it will successfully serialize in/ out of JSON. However, when an op1 is a vector of symbols (i.e. a composed operator), it will be parsed in as a Symbol("[\"d\",\"d\"]"), which is not the desired behavior.

This problem can be immediately resolved by calling `expand_operators` before `write_json_acset`, but a better solution would overload `read_json_acset` for the special case of a vector of symbols.