Fixing a few small bugs

* if a filename is passed from a script and had a blank at the very end,
then the file extension (including the blank) was not properly
recognized -> Solution: add a deblank on the extension.
* Fixing the estimation of power spectrum:
- ensuring 1st and last (4sec) window are processed like the others
- before power spectrum estimation (pwelsh)
- filtering between .01Hz and fmax+1Hz done in 2 steps (high- then
low-pass) instead of a single bandpass
- then removing the mean of filtered signal over window
* Fixing small issue when displaying the power spectrum, incl. the
hypnogram
* Fixing again the 'add atefact on this channel' function. It should now
be using and saving the right channel index.