GroupingGetterFunction should be allowed to return arbitrary value (#957

)

Underlying slickgrid logic only takes into account equality between values
returned for grouping individial rows. Getter thus does not need to
return `string` but might return arbitrary value (e.g. string, number, Date).

src/models/grouping.interface.ts

@@ -3,7 +3,7 @@ import { GroupingComparerItem } from './groupingComparerItem.interface';
 import { GroupingFormatterItem } from './groupingFormatterItem.interface';
 import { SortDirectionNumber } from './sortDirectionNumber.enum';
 
-export type GroupingGetterFunction<T = any> = (value: T) => string;
+export type GroupingGetterFunction<T = any> = (value: T) => any;
 
 export interface Grouping<T = any> {
   /** Grouping Aggregators array */