DOC: document the units for the conversion utils
DOC: document the units for the conversion utils

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