MNT: Handle image length if an iterable is provided
MNT: Handle image length if an iterable is provided

If an iterable is provided, then len(iterable) might raise. This fixes
that potential problem by just returning the number of images as '-1'

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