steinhart_hart: Add Steinhart-Hart eq fns

- Port from thermostat firmware

src/thermostat/mod.rs

@@ -1,4 +1,5 @@
 pub mod ad5680;
 pub mod max1968;
 pub mod thermostat;
-pub mod ad7172;
+pub mod ad7172;
+pub mod steinhart_hart;

src/thermostat/steinhart_hart.rs

@@ -0,0 +1,40 @@
+use num_traits::float::Float;
+use uom::si::{
+    f64::{
+        ElectricalResistance,
+        ThermodynamicTemperature,
+    },
+    electrical_resistance::ohm,
+    ratio::ratio,
+    thermodynamic_temperature::{degree_celsius, kelvin},
+};
+use miniconf::Miniconf;
+
+/// Steinhart-Hart equation parameters
+#[derive(Clone, Debug, PartialEq, Miniconf)]
+pub struct Parameters {
+    /// Base temperature
+    pub t0: ThermodynamicTemperature,
+    /// Base resistance
+    pub r0: ElectricalResistance,
+    /// Beta
+    pub b: f64,
+}
+
+impl Parameters {
+    /// Perform the voltage to temperature conversion.
+    pub fn get_temperature(&self, r: ElectricalResistance) -> ThermodynamicTemperature {
+        let inv_temp = 1.0 / self.t0.get::<kelvin>() + (r / self.r0).get::<ratio>().ln() / self.b;
+        ThermodynamicTemperature::new::<kelvin>(1.0 / inv_temp)
+    }
+}
+
+impl Default for Parameters {
+    fn default() -> Self {
+        Parameters {
+            t0: ThermodynamicTemperature::new::<degree_celsius>(25.0),
+            r0: ElectricalResistance::new::<ohm>(10_000.0),
+            b: 3800.0,
+        }
+    }
+}