changes mri/core.py weight -> weights
changes mri/core.py weight -> weights

this corrects a bug where mesh.from_image wouldn't run because the
kwarg was weight, instead of weights. this changes all the arguments
to weights in order to make it consistent with geometric/mesh.py,
where the argument / keyword weights seems way more common than here

noahbenson/neuropythy,noahbenson/neuropythy