fixed units issue with Astropy being upset over None
fixed the len(f)/nested f issue; smooth_grid was passing wrong array

smooth_grid was passing the full model['fsyn'], instead of the indexed
model['fsyn'][i], so variable_smooth and falt2 were getting confused
by the nested array (and that's why my usual nesting tests didn't
work - it wasn't a length-1 array, it was an array of arrays.