lisa.datautils: Make series_rolling_apply(center=True) center on the window

It currently centers on the passed series' index rather than on the window
center, which can be somewhat confusing:

    window center
	  v
	  v
    [     | | |]
	    ^
	    ^
       series center

| = event in the series
[ = window start
] = window end

Suggested-by: Douglas Raillard <douglas.raillard@arm.com>