ENH: Added check when DataSet constructor gets path to the file, which is not a correct DataSet archive.

ENH: Added data check before creating a new DataSet - prevents SEGFAULTs occuring when read() method was not called beforehand.

ENH: Now it's possible to specify different neuron types for different hidden layers in FullyConnectedFFNs.

ENH: Methods print_* changed for overloaded methods write_*, which make possible writing not only to STDOUT, but also to the file.