Update javadoc of the groupByEntity method

[bonaparten]

Bug Description

The method groupByEntity in the MapReducer class throws an UnsupportedOperationException with the following message: groupByEntity() must be called before any map or flatMap transformation functions have been set if it is called after map or flatMap functions. The javadoc of the method informs that This needs to be called before any map or flatMap transformation functions have been set. However, the exception is thrown even if groupByEntity is called after a filter function (or maybe also after other functions?).

Expected behaviour

The exception and the javadoc should mention for what other functions the method needs to be called before.

System information

• OSHDB Version: 0.6.4