Add "Group by Process" option to frontend.

AT:

• All report types that can be grouped by process (everything except raw data, variable and incident reports) have a "group by process" option either in the groupby or distributed by dropdown
• Median duration aggregation is disabled if the report is grouped by process
• A tooltip explains why the median duration aggregation is disabled
• The group by process option is only available for reports with multiple datasources and should not be displayed otherwise
• If datasources are removed so that the report contains only a single datasource, any group by process selection is automatically removed