Move model parser to separate class

[StefSchenkelaars]

I wanted to make some structural changes to the library. As explained in #72 I want to simplify the configuration. But next to this I also want to separate the business logic (the parsing) from the domain objects (the Model).

My proposal therefore is to have separate parsing classes (like the ModelParser in the commit) and separate domain objects (Model). This way we can separate the different concerns:

• Read the files and comments (YARD)
• Transform the YARD objects and tags into domain objects (Parsing)
• Transform the domain object in Swagger / OpenAPI objects (Transforming)

Let me know if you like this direction, then I could extend this to all domain objects and start separating the steps.