move table columns mixin to mixins.less

less/mixins.less

@@ -319,6 +319,15 @@
   }
 }
 
+// Table grid system
+// -------------------------
+.tableColumns(@columnSpan: 1) {
+  float: none; // undo default grid column styles
+  width: ((@gridColumnWidth) * @columnSpan) + (@gridGutterWidth * (@columnSpan - 1)) - 16; // 16 is total padding on left and right of table cells
+  margin-left: 0; // undo default grid column styles
+}
+
+
 // Make a Grid
 // -------------------------
 // Use .makeRow and .makeColumn to assign semantic layouts grid system behavior