engine: heat in a fluid step

A fluid domain takes `thermal` settings (the initial temperature, a
thermal condition per box face and per obstacle face, gravity with the
expansion coefficient for buoyancy) in model.fluid_domain.create and
.edit, and model.fluid_domain.thermal_boundary sets one boundary's
condition as model.fluid_domain.boundary does for the flow. A material
a domain with heat uses keeps needing its conductivity and specific
heat when it is replaced.

solve.fluid's summary adds the thermal relaxation time, the fluid's
temperature range and the heat into every wall (final and the mean
over the last quarter of the history); solve.probe gives the
temperature in the cells at points and, with `heat`, the walls' heat
histories. The form, the catalogue and docs/features.md follow.

Part of #157.

crates/engine/src/cmd/mod.rs

@@ -170,6 +170,7 @@ pub static ALL: &[CommandSpec] = &[
     define::FLUID_DOMAIN_EDIT,
     define::FLUID_DOMAIN_DELETE,
     define::FLUID_BOUNDARY,
+    define::FLUID_THERMAL_BOUNDARY,
     define::STEP_CREATE,
     define::STEP_EDIT,
     define::STEP_DELETE,