Refactor DataExtractionTransformer.java

[joker234]

#75 introduced the new class DataExtractionTransformer.java. This was a hotfix fixing the non-serializability of some classes making the ohsome API's /elemensFullHistory endpoint unable to work with ignite.

The new class has to be refactored, especially:

• Add javadoc
• Refactor variables, input parameters of the constructor
• Fix other Issues (e.g. from SonarLint)
• Unit Tests?

[tyrasd]

most of this should be addressed with #178. proper dedicated unit tests would be nice, but from what I can see, the integration tests already cover this class quite well: https://jenkins.ohsome.org/job/ohsome-api/job/master/515/jacoco/org.heigit.ohsome.ohsomeapi.executor/DataExtractionTransformer/