Make group names optional?

[nathanathan]

I think we could make things a tiny bit easier on form designers by auto-generating group names when they aren't in the XLSForm. It would be a pretty small code change, we just need to just uncomment these lines to add it:

https://github.com/UW-ICTD/pyxform/blob/f165f8acb808771bbd6a79ac873f47730596c9ec/pyxform/xls2json.py#L635

@ukanga you have a test that asserts leaving a group unnamed should raise an error, so I'm wondering if you think this would be a bad idea:
https://github.com/modilabs/pyxform/blob/master/pyxform/tests/bug_example_xls/group_name_test.xls