divide by zero in volume fraction normalization of tortuosity

[matteofrigo]

When we pass the volume fraction of both the intra and the extra cellular compartments to the function that computes the perpendicular diffusivity through the tortuosity constraint, there could be a division by zero that breaks everything.

https://github.com/AthenaEPI/dmipy/blob/9bd7038ea93edbf9cced56a7ef6a0f19f25388c3/dmipy/utils/utils.py#L231

We might want to catch it and avoid the division in that case. Any more sophisticated thoughts @rutgerfick ?

[rutgerfick]

No you're completely right, this is something that should be caught if it happens. How about adding a small epsilon in the denominator to avoid this? something like vf_intra = vf_intra / (vf_intra + vf_extra + eps)?

[matteofrigo]

I've never liked that trick in general. In this context it may work since the parameter range for volume fractions is [0.01, 0.99] and I guess that in the optimization the variable is projected in that interval (correct me if I'm wrong), but I feel like it's not as clean as

denominator = vf_intra + vf_extra
if denominator == 0:
    vf_intra = 0
else:
    vf_intra = vf_intra / denominator

which comes almost for free.

[rutgerfick]

fair enough, please go ahead and add it and I'll merge.