Add option to truncate leading or trailing block

[jjhbw]

Leading block usually presents with an attenuated signal due to acclimatisation effect of the participant.
See figure.

[jjhbw]

Update: this is likely an issue with the way the (example) timeseries data were prepared. In this case, the FEAT model consisted of a basic on-off shape convolved with a gamma function, leading to a weird nudge at the first on-block. The example timeseries in the image fits this convolved function well, but it may not be what we want. I think this issue is best resolved before timeseries extraction, so beyond the scope of this repo.