Move check for mask=None and update doc for phi to reflect Julien's suggested x=col y=row convention
Move check for mask=None and update doc for phi to reflect Julien's suggested x=col y=row convention

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