MNT: Wrap pyfai import in try/except block
MNT: Wrap pyfai import in try/except block

pyFAI is not python 3 compatible, so we need to wrap its import in a
try/except block and raise a RuntimeError in functions where it is used
if it is unimportable

scikit-xray/scikit-xray,celiafish/scikit-xray,CJ-Wright/scikit-beam,CJ-Wright/scikit-beam,hainm/scikit-xray,danielballan/scikit-xray,tacaswell/scikit-beam,licode/scikit-beam,tacaswell/scikit-beam,licode/scikit-xray,danielballan/scikit-xray,yugangzhang/scikit-beam,scikit-xray/scikit-xray,licode/scikit-beam,scikit-xray/scikit-xray,danielballan/scikit-xray,hainm/scikit-xray,tacaswell/scikit-xray,Nikea/scikit-xray,CJ-Wright/scikit-beam,licode/scikit-xray,yugangzhang/scikit-beam,celiafish/scikit-xray,Nikea/scikit-xray,yugangzhang/scikit-beam,celiafish/scikit-xray,ericdill/scikit-xray,ericdill/scikit-xray,licode/scikit-beam,tacaswell/scikit-beam,hainm/scikit-xray,ericdill/scikit-xray,Nikea/scikit-xray,licode/scikit-xray,tacaswell/scikit-xray,tacaswell/scikit-xray