smurf: Allow mean time slice values to be subtracted before forming Q and U
smurf: Fix nasty bug in pca cleaning code

An unterminated comment meant that gaps were not being filled.
smurf: Take account of iterator differences in python 2.7 and 3
smurf: Fix handling of REF in skyloop

Previously, if a REF image was supplied it was used as the initial sky
estimate on every iteration.