Set default activation of committor model back to tanh

[EnricoTrizio]

After #135, the activation of hidden layers of the Committor class is no longer set to tanh as the default, which should be the proper choice.

For the moment, this can be circumvented by setting

options = { 'nn' : { 'activation' : 'tanh' }}
model = Committor(..., options=options, ...)

This should be fixed soon.