hggroup, specgraph.contourgroup moved

Moved both to a more logical place: together with the other
cases that call `handleAllChildren`
Default handling: use back-tracking

When an unknown handle graphics element is encountered,
execute the default handling using back-tracking.
Whenever an error occurs, roll back the state of `m2t` to before
encountering the hg-object and continue with the next element.

The original error message is shown for debugging purposes, but
this allows one to run `matlab2tikz` on figures that have
unsupported elements, as was the case in Issue273
Merge pull request #275 from egeerardyn/fixIssue273

Fix issue273