Export altitude from LandXML coordinates

lib/cg-point.js

@@ -1,4 +1,6 @@
 import { getProp, parseSequence, getColor } from './util'
+import { polygon } from '@turf/helpers';
+import union from '@turf/union';
 import proj4 from 'proj4';
 
 export const LANDXML_POINT = 'cg_point';