Add method to retrieve cell temperature
Created by: aprilnovak
For simulations with thermal-hydraulics feedback in Picard-style iteration loops, convergence of the coupled solve may be based on the change in a temperature norm between two successive iterations. If OpenMC runs first in the Picard iteration loop, the change in the norm for the very first iteration should compare the most recent T/H solve to the temperatures used in the OpenMC input file.
It would be helpful to add a new method with a signature like:
extern "C" int openmc_cell_get_temperature(int32_t index, double& T, const int32_t* instance)
that will allow a coupled driver to determine what temperatures were actually used as the initial condition in OpenMC.
The alternative would be to separately inform a multiphysics driver (i.e. through an XML input file) of the OpenMC initial condition, which 1) is error prone for the user and 2) requires additional development effort on the multiphysics driver side unless you're doing something simple, like setting a uniform temperature in the entire OpenMC model.