Fix downsampling frequency

[cedricfoucault]

Follow-up to the changes in #32 : The sampling frequency of the preprocessed data has been changed from 1kHz to 250Hz. As a result, because the TRF scripts currently downsample by a factor of 10, the TRFs resolution have gone down from 100Hz to 25Hz.

This change is to modify the TRF scripts so that they adapt how the downsampling so that the target sampling frequency remains 100Hz regardless of the original sampling frequency.