Implement Havenith (2001) two-node model

[FedericoTartarini]

Implement the two-node model described in this paper:

@article{havenith_individualized_2001, title = {Individualized model of human thermoregulation for the simulation of heat stress response}, volume = {90}, issn = {8750-7587}, url = {https://journals.physiology.org/doi/full/10.1152/jappl.2001.90.5.1943}, doi = {10.1152/jappl.2001.90.5.1943}, number = {5}, urldate = {2023-09-05}, journal = {Journal of Applied Physiology}, author = {Havenith, George}, month = may, year = {2001}, keywords = {body fat, body temperature, exercise, fitness, heat strain}, pages = {1943--1954}, }