2.3.3

• sigma now has a different decay function and it can be selected via the parameter sigma_decay_function
• the user is now warned when sigma is higher than sqrt(x^2+y^2)
• a selection of decay functions for both sigma and learning rate has been implemented by @BrandonGarciaWx
• the train method now accept fixed_points so that the algorithm can be force to train specific neurons for specific samples