Fix for #634 (#638)

squashed commits as some the earlier commit were later reversed

* Added exposure adjustment to barnzehn.py. Updated ptf example gallary with exposure adjustments to match paper.

* fixed bug when not passing sample_weight to BarnettZehnwirth.fit. updated plot_ptf_resid in gallery.

* removed sample_weight from barnettzehnwirth.fit

* unremoved sample_weight from barnettzehnwirth.fit -- the estimator ignores it as it did originall

* fixed graph title

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Co-authored-by: danielfong-act <[email protected]>

Author: danielfong-act