DigiOutputFiles created without readoutmode attribute

[chiaratomaiuolo]

In this moment, in the class DigiOutputFile* (where * indicates all the 3 possible readout modes) the attribute readoutmode is not initialised in the file header, instead, this attribute is copied in the header in hxsim.py through the readout_args variable, that define which HexagonalReadout* to use. The correct way is initialising immediately the attribute without writing it twice in the file header.