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},
}