swa_get_peaks fails to handle a special case

[grahamfindlay]

Checking for negative and positive peaks this way...

MNP  = find(diff(sign(slope_data)) == 2);
MPP  = find(diff(sign(slope_data)) == -2);

...fails to properly handle the case where the slope at the peak is exactly zero.

E.g. it catches peaks like / but misses peaks like _/.