Saving of mat files is corrupted

In the latest branch (feat/new_architecture), the mat files saved pre- and post-optimisation in `save_diff_gfdn_parameters` (in save_results.py) are corrupted because I cannot open them with loadmat.
It is possible that registering some buffers in model.py is causing this, eg: `self.register_buffer('delay_buffer', self.delays)`
