Grouping and summary has been revamped in this version to support merging of grouped cells,
export of grouped data, extend keyboard navigation to grouped cells like Excel,
allow editing of summary cells, display grand summary in the bottom fixed row,
override or extend the default rendering with
callback and various other options.
There have been following changes in the API.
column.summary.type is string instead of array.
column.summary.title is replaced by more generic
groupModel.icon is single dimensional array.
groupModel.summaryCls are unsupported.
column.render to inject classes.
groupModel sub-options should be changed only through