Store GUFE version information when serializing to/from dictionary

[IAlibay]

Use case

It would be really useful to know what version of gufe a JSON was written with so that you can easily say "if I load it back with this version of gufe and compatible version of openfe/feflow/whatever-else then it should work".

The proposal

We store the gufe.__version__ when calling to_dict.

This is done at to_dict call no matter what the gufe version is, i.e. if you load an old JSON file serialized with an old version of gufe, and it works in a new version of gufe and you then to_dict again, then the new version of gufe is written out to the JSON file.