Changed behaviour of nisl.signals.clean()

The "standardize" parameter of nisl.signals.clean() is now used for setting to
unit variance the *output* signals, instead of the *input* before confounds
removal. This latter operation is now performed in all cases. This standardization
is robust to signals that are (almost) zero everywhere.

This modification may have a lot of consequences. It has been made because standardizing the output of clean() was required for plot_regions_covariance.py in the "roi" branch. The new behaviour is technically consistent, but I'd like to have everyone's opinion before merging into "roi" (or master).