Code clean-up: refactor CodegenResponse, CodegenProperty and CodegenParameter

[jmini]

CodegenResponse, CodegenProperty and CodegenParameter are very similar, but for the moment they to not share nothing.

With OAS2 specification, it made sense to have three objects because every of these concepts was different in the spec. Now with OAS3, everything is Schema based. But in our methods fromModel(..), fromProperty(..) and fromResponse(..) reimplement everything three times…

The first step will be to identify what is common between CodegenResponse, CodegenProperty and CodegenParameter. A new abstract Codegen should be created: AbstractSchemaBasedCodegen (or something like that). It contains the common stuff and it is derived from the schema.

Then we will be able to remove the redundancy in DefaultCodegen and we will have something that works the same way for consumers of CodegenResponse, CodegenProperty and CodegenParameter.

[jmini]

As a first step, we should ensure consistency between the properties of each Codegne. For example:

• org.openapitools.codegen.CodegenModel.dataType
• org.openapitools.codegen.CodegenParameter.dataType
• org.openapitools.codegen.CodegenResponse.dataType
• org.openapitools.codegen.CodegenProperty.datatype

datatype should be renamed dataType in the code and in the templates => #69

[jmini]

An other related topic, we need access to the openAPI instance, see #83

[jmini]

Discussed in #75, CodegenModel needs isInteger similar to CodegenProperty.

[cvgaviao]

As a first step, we should ensure consistency between the properties of each Codegne. For example:

* `org.openapitools.codegen.CodegenModel.dataType`

* `org.openapitools.codegen.CodegenParameter.dataType`

* `org.openapitools.codegen.CodegenResponse.dataType`

* `org.openapitools.codegen.CodegenProperty.datatype`

datatype should be renamed dataType in the code and in the templates => #69

Hi, could you explain me please the reason those classes do not have a complexType property as has a CodegenProperty (in other to have both the base collection type and its inner type)?