DataTag ctor throwing errors

[balint42]

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.