Remove underscore in activation functions

[EnricoTrizio]

In a future release, should we remove the underscore from custom activation classes Custom_Sigmoid and Shifted_Softplus?

From #135

I see, thanks. In principle, if Shifted_Softplus was in a previous release, we'd have to deprecate it and remove it only when we'll release 2.0. If that's the case, we could just change their name and do

class Shifted_Softplus(ShiftedSoftplus):
    def __init__(...):
        warn('Shifted_Softplus is deprecated use ShiftedSoftplus instead.')
        super().__init__(...)