model: heat in a fluid domain

A fluid domain may carry thermal settings: the initial temperature,
gravity and the fluid's expansion coefficient for buoyancy, and a thermal
condition (adiabatic, held temperature, heat flux) on each box face and on
named obstacle faces, adiabatic unless told otherwise. The material then
needs an isotropic conductivity and a specific heat. Boundaries are set
one at a time like the flow ones.

Part of #157.

crates/lbm/tests/common/mod.rs

@@ -21,6 +21,7 @@ pub fn domain(min: [f64; 3], max: [f64; 3], spacing: f64) -> FluidDomain {
         initial_velocity: [0.0; 3],
         acceleration: [0.0; 3],
         tolerance: None,
+        thermal: None,
     }
 }