Change 'groupByObject" to 'groupByObjectId' in JSON response

[bonaparten]

The value of groupByObject in a JSON response represents the ID of the object, not the object itself.
groupByObject should be changed to groupByObjectId or something similar.

[FabiKo117]

Relevant code line:

ohsome-api/src/main/java/org/heigit/ohsome/ohsomeapi/output/dataaggregationresponse/groupbyresponse/GroupByObject.java

Line 21 in 73ce61a

protected Object groupByObject;

in class GroupByObject

[tyrasd]

see also 360508a, #57 (comment) and #80

[tyrasd]

what would you do with the multi-groupBy requests, like /groupBy/boundary/groupBy/tag/ for example. currently, those look like this:

    "groupByObject" : [ "Heidelberg", "highway=tertiary" ]

Would you name those groupByObjectIds or groupByObjectsIds perhaps?

[FabiKo117]

We've decided in a 2.0 release meeting that we want to call this response object simply group for all the /groupBy responses.