Resampling files as they are loaded

[cwbishop]

At present, SIN_loaddata only loads data, but it may be wise to have it handle the resampling procedure as well to ensure that all stimuli are loaded/resampled in the same way across all functions.

This became an issue when writing modifier_append2playlist, which loads the stimuli and appends them to an already existing cell array of (resampled) data series.