Rename and restructure classes and packages

[bonaparten]

A general review of all classes' and packages' names and structures would be useful.

For example, AggregateRequestExecutor.java and DataRequestExecutor.java could be renamed to AggregationRequestExecutor.java and ExtractionRequestExecutor.java. So these classes will follow the same naming criteria used in other packages, i.e. in the package output: DefaultAggregationResponse.java and ExtractionResponse.java .

[KowatschFab]

I agree, it would definitely make sense to review again the current structures and names. After we've merged #124 and #121 we can think about which other classes and packages would need to be renamed/restructured.

[bonaparten]

I pushed this old commit. It is an example of how we can restructure some packages and classes.