FEHM source code modifications and executables for use with ocean-world gravity

This is a repository for compiled codes, source code, and input files used
in this paper: Fisher, A. T., K. D. Dickerson, D. K. Blackman, N.
Randolph-Flagg, C. R. German, and C. Sotin (2024), Sustained hydrothermal
circulation under ocean-world gravity, J. Geophys. Res. - Planets,
submitted and in review. Plain language summary from paper: Ocean
worlds are planetary bodies that have a liquid ocean, often under an icy
shell or within the rocky interior. In Earth's solar system, several
moons of Jupiter and Saturn are ocean worlds. Some ocean worlds are
thought to have hydrothermal circulation, where water, rocks, and heat
combine to drive fluids in and out of the seafloor. Hydrothermal
circulation would impact the chemistry of the water and rock of ocean
worlds and could help life to develop deep below the icy surface. This
study shows results from computer simulations of hydrothermal circulation,
based on a well-understood system on Earth, to measure the influence of
lower gravity like that appropriate for ocean worlds smaller than Earth.
The simulations with ocean world (lower) gravity result in fluid
circulation much like that occurring on and below Earth's seafloor,
but with several important differences. Lower gravity reduces buoyancy,
meaning warmed fluids don't become as light when heated. Lower
buoyancy tends to reduce flow rates in a hydrothermal system, and this
raises the temperatures of the circulating fluid, which would allow more
extensive chemical reactions. Lower flow means less heat transport, and
this could help these flows last longer in an ocean world.