Fixed #647 - DataTable sortField allows a function, but the prop type is only a string

src/components/datatable/DataTable.d.ts

@@ -16,7 +16,7 @@ export declare class DataTable extends Vue {
     lazy?: boolean;
     loading?: boolean;
     loadingIcon?: string;
-    sortField?: string;
+    sortField?: string | Function;
     sortOrder?: number;
     defaultSortOrder?: number;
     multiSortMeta?: any[];

src/components/datatable/DataTable.vue

@@ -207,7 +207,7 @@ export default {
             default: 'pi pi-spinner'
         },
         sortField: {
-            type: String,
+            type: [String, Function],
             default: null
         },
         sortOrder: {

src/components/treetable/TreeTable.d.ts

@@ -21,7 +21,7 @@ export declare class TreeTable extends Vue {
     loadingIcon?: string;
     rowHover?: boolean;
     autoLayout?: boolean;
-    sortField?: string;
+    sortField?: string | Function;
     sortOrder?: number;
     defaultSortOrder?: number;
     multiSortMeta?: any[];

src/components/treetable/TreeTable.vue

@@ -166,7 +166,7 @@ export default {
             default: false
         },
         sortField: {
-            type: String,
+            type: [String, Function],
             default: null
         },
         sortOrder: {