Shift time by average time (t → t-t₀)

This is supposed to reduce parameter correlations, but I'm not
conviced it works when you have a shape that is symmetric in the
observable, e.g. straight line.  Manuel cited track fits, I agree it
works there but that's because they have 2 observables.

Update: showed no improvement to linear ratio fit with this change
Bug fix: flat ratio

Flat ratio was fixed to 1.0 instead of fitting for the overall scale
factor.
Stop fitting for offset in linear x-check (set to constant)

Instead of fitting for it, set it to some reasonable value (0.9).  1.0
is probably too close to perfect, then the striaght line slope does
not have any wriggle room.

FIXME: doesn't work
Add quadratic acceptance ratio (doesn't work)
Stricter slope range for linear x-check