FIX: Removed 'normalized' and 'de-normalized' evaluation as there is just 'de-normalized' natively from now.

FIX: fixed an issue with NaN in GradientDescent caused by acos(x), added more debug info to errorfunction validation

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.