Defining activation functions in several places

[rmisener]

We currently define the activation functions for neural network nodes in several places:

https://github.com/cog-imperial/OMLT/blob/main/src/omlt/neuralnet/layer.py --> Line 112 - 122
https://github.com/cog-imperial/OMLT/blob/main/src/omlt/neuralnet/activations/smooth.py --> Lines 4 - 43

This is a problem because anyone adding a new activation function (or debugging an existing activation function) will have to add the new activation function twice. Additionally, softplus seems to be missing from one of the two places where we define activation functions (https://github.com/cog-imperial/OMLT/blob/main/src/omlt/neuralnet/layer.py).