Introduce semantic validation of swagger doc

[giuliopulina]

Introduced semantic error validation (#4):

Definitions:

• A definition/model cannot declare a property that is already defined by one of its ancestors.
• Discriminator must be defined as a required property
• Discriminator must be an existing property of definition/model
• Required properties must be declared as properties in the definitions
• Required properties cannot contain duplicates
• References to other definitions must exist
• "Items" property must be defined in array

Operations:

• Operation cannot contain both form and body parameter
• Operation must contain at most one body parameter
• Api path must be unique (not equal/equivalent to another defined path)
• Path params must by matched by operations params (and viceversa)
• Operation must not duplicate parameters (combination of "name" and "in" must be unique)

[giuliopulina]

Thanks for the suggestion: you are right, I generated with IntelliJ the delegate with inheritance when I wasn't 100% sure about the approach (and the fields I needed). I just pushed a new commit removing inheritance and useless methods.