DataTag ctor throwing errors

Currently the `DataTag` ctor will throw errors if an invalid `DataTag` is opened: it uses the `positions` method to set the supplied obligatory `DataArray` as `positions` array. This is not a good situation, as the error will obstruct every other operation (e.g. validation) one could want to perform on the `DataTag`. Thus we should perhaps consider not doing any kind of check in the ctor and set the supplied `DataArray`'s `id` no matter what.