First attempt to approach the issue bob.bio.base#106

In this issue I'm:

1. Removing the loading and saving functions via HDF5 in the C++ and in
the python API (not sure if it's really necessary to remove from the C
level)

2. Implemented the methods `to_dict` and `from_dict` for the activation
functions

3. Test cases
parent 9b9dec5e
Pipeline #32409 passed with stage
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