Set default statistic in __init__ and allow override in __call__.
Set default statistic in __init__ and allow override in __call__.

tacaswell/scikit-xray,scikit-xray/scikit-xray,tacaswell/scikit-xray,tacaswell/scikit-beam,Nikea/scikit-xray,scikit-xray/scikit-xray,danielballan/scikit-xray,scikit-xray/scikit-xray,Nikea/scikit-xray,tacaswell/scikit-beam,tacaswell/scikit-beam,tacaswell/scikit-xray,Nikea/scikit-xray,danielballan/scikit-xray,danielballan/scikit-xray