constraints on groups

[MartijnR]

Issue seen as:
'field-list' entered as a constraint on a group (instead of as an appearance) is passed through pyxform without errors and in the resulting XML it is added as a constraint to the binding of that group.

This may not cause errors in clients such as ODK Collect (though not tested by me), because constraints on groups do not exist in javarosa, as far as I know, and are therefore never evaluated. However, a user will not get feedback about using the incorrect column and it may be hard to figure out why a group is not shown as a field-list without that feedback.

It may be better to provide a warning message when a constraint is added to a group (or to a repeat in fact as that is not supported either).