Listspikes improved

Also:
simplified aamod_LoAd
added aamod_ggmfit (using suggestion by C. Beckmann to normalise
T-value distribution in contrast)
allowed second level model wo use T-values (necessary for ggmfit)
improved aamod_mask_fromstruct diagnostics & basic mask
Changed aamod_firstlevel_model

Modularised this to make it more manageable