`.htg` format is too XML wordy

Currently, when one saves an htg into a .htg file, there is one XML element definition per hypertree. In the case of htgs with millions of trees, the file becomes outrageously large (in the range of gigabytes when an unstructured grid equivalent only weights megabytes).

Every information necessary to construct hypertrees should be stored into field data arrays in order to fix this issue.