Expand operators before writing JSON

[lukem12345]

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.

[olynch]

With intertypes, we can serialize an attribute that is a vector of symbols.