add parameter description in aggregation responses

[bonaparten]

Description

Add descriptions of the response parameters in the docs.

Corresponding issue

#88

New or changed dependencies

none

Checklist

• My code follows the code-style rules, and I have checked on the static analyses
• I have commented my code
• I have written javadoc (required for public methods)
• I have added sufficient unit and API tests
• I have made corresponding changes to the documentation
• I have updated the CHANGELOG.md
• I have adjusted the examples in the check-ohsome-api script or created an issue in the corresponding repository. More Information here.

[FabiKo117]

Can we potentially shift this PR to 2.0 and do #16 first, as there would be a change in the structure and therefore also of some response objects?

I guess those response parameters are not thaaat important, that they would be needed in the next minor release, so they could also wait until the next major release.

[bonaparten]

Can we potentially shift this PR to 2.0 and do #16 first, as there would be a change in the structure and therefore also of some response objects?

I guess those response parameters are not thaaat important, that they would be needed in the next minor release, so they could also wait until the next major release.

No problem with that. We can wait until we solve #16.

[bonaparten]

#194 solves #88.