Replace circularity with sphericity

[AoifeHughes]

microCT_grain_analyser/MATLAB/countGrain.m

Line 100 in d1bf5a0

stats.circularity = 4 * pi * (st.Area / st.Perimeter ^ 2);

I don't have matlab currently or data to test, but something like
would give a value where grains are approximated to ellipsoids based on their length, width depth. (l,w,d would need to be halved to be their abc lengths).

[AoifeHughes]

Above is just eq for sphericity, top line is surface area of sphere of similar volume (V=v of ellpsoid of l,w,d) over surface area of ellipsoid l,w,d. This would also be nice as it ignores rotation of grain.