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
2 jobs for big-issue-106 in 20 minutes and 1 second (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #170441
docker
build_linux_36

00:17:45

88.0%
passed #170442
macosx
build_macosx_36

00:20:01

88.0%