Bumps when loading custom rates

Interpolating over the rate table seems to be adding bumps in the rate (this is the issue reported by Francisco De Geronimo)

For the attached triple alpha rate see the plot for what happens at low T. Sure the rate has only 23 points so is under-sampled but I would have expected a smoother curve from MESA.


Input rate
[r_he4_he4_he4_to_c12.txt](https://github.com/MESAHub/mesa/files/8869676/r_he4_he4_he4_to_c12.txt)

MESA's cached rate

[r_he4_he4_he4_to_c12_0.txt](https://github.com/MESAHub/mesa/files/8869678/r_he4_he4_he4_to_c12_0.txt)

Maybe its an issue with having only a few points near the boundary? As at higher T's the curve is smooth.

Start by reproducing the low-temperature bumps using the attached input rate and MESA's cached rate, then inspect the Fortran code responsible for loading and interpolating custom rates. Done means identifying and correcting the interpolation behavior so the cached curve is smooth at low temperatures while preserving the higher-temperature behavior.

Written by the indexing model from the issue text.