activation intrinsics for neural networks

[jalvesz]

Add activation functions for neural networks. Relates to #858

Todo:

• specs & autodoc
• tests
• examples

Previous Art
SciPy Special functions collection: https://docs.scipy.org/doc/scipy/reference/special.html
torch nn: https://pytorch.org/docs/stable/nn.html#non-linear-activations-other
neural-fortran : https://github.com/modern-fortran/neural-fortran/blob/main/src/nf/nf_activation.f90

cc: @Beliavsky @milancurcic @epagone @jvdp1 @perazz

[jalvesz]

Before moving forward, any opinions on

• putting these functions within the "specialfunctions" category?
• naming of the derivative/gradient version with <name>_grad?
• any other remark?