Using Julia files instead of Yaml files

Despite I like Yaml files, I think it would be easier just to use a Julia file as an input. That would:

- remove one dependency
- having lbxflow in the execution path
- take benefit of the type system (i.e. providing the inputs in the right format)
- it doesn't feel like it will be more verbose (in fact, the `expr = false` won't be necessary)

Just a suggestion
