Activation function specification alignment

[SkBlaz]

Hello, whilst inspecting the nn part, noticed the following discrepancy, perhaps worth noting:

https://github.com/EpistasisLab/tpot/blob/c3a155b5a76f7ac0911482d0478d4ca6c2fcb3bd/tpot/builtins/nn.py#L218

The suggested solution is aligned variable/activation naming (e.g. self.tanhAct)