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.

ENH: Added method for evaluation errors on an arbitrary data-set. Several methods overloaded to be able to output into the file according to the open handler.

ENH: Method run_k_fold_test overloaded, so that it can output not only to STDOUT, but also to the file, either according to its name or to the open handler.

FIX: Removed include of the exception header - it does NOT belong here, as it's not supposed to be used in the API!