Thermal expansion correction of fuel height

------------------------------------------------------------------------------------

PyDrag is primarly based on a set of IRSN calculation procedures, written in CLE-2000. An extract of this reference can be found on this GitHub repository Therefore, these procedures have been used as the main reference for the PyDrag development, especially on the non-regression tests done by developers. This issue highlights a known approximation that the user should be aware of.

------------------------------------------------------------------------------------

Several comparisons have been made on GridDil.c2m procedure from IRSN reference and grid dilution process in PyDrag. It has been shown that the fuel height thermal expansion is never taken into account during the calculation of total assembly volume of few non-Tihange assembly models. However, this same data is corrected when it is used to calculate total fuel volume in assembly (c.f. CreaMix.c2m).

The GridDil.c2m procedure comes from the grid dilution procedure from BEAVERS - PYTHRA2018. Further inverstigations done on IRSN private archives showd that this value was initially corrected in the original BEAVERS grid dilution process and has been deleted few commits later (probably by mistake).

This mistake does impact the kinf calculation for about 1 pcm. For non-regression purpose, it is kept during the developpement of PyDrag and should be fixed later.