CogniCrypt_GEN - Get rid of addReturnObject method

[kruegers]

Is your feature request related to a problem? Please describe.
The fluent interfaces of CogniCrypt_GEN allow calls to three methods for configuration: includeClass, addParameter, and addReturnObject. The last one associates the passed variable from the glue code with the return object of the last call that is generated for the given class that has a return object (of fitting type). That's quite indirect and hard to wrap one's head around.
Fortunately, addReturnObject should not actually be necessary. Method addParameter links a variable from the glue code with a variable from the respective CrySL rule. Return objects of any relevance should be specified as extra CrySL variables anyway. Therefore, all calls to addReturnObject(a) should be replacable with calls to addParameter(a, retVarInCrySLRule).

Describe the solution you'd like
Investigate all existing code templates and, in particular, calls to addReturnObject for whether or not it is possible to apply the above fix. If so, perform this replacement. Either remove addReturnObject entirely from the API (and its backend support) or, at the very least, deprecate it.

Describe alternatives you've considered
Keeping it. But no.

[kruegers]

assigned @rakshitkr.

[rakshitkr]

@kruegers Please review my changes