Add linear approximation and use it for improving battery percentage

Add linear approximation class and use it to better model the non-linear
discharge curve of the battery.

Changed the minimum voltage level to 3.5V and the maximum to 4.18V. For
reference the maximum observed voltage is 4.21V during charging.

src/components/utility/LinearApproximation.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <cstddef>
+#include <array>
+
+namespace Pinetime {
+  namespace Utility {
+
+    // based on: https://github.com/SHristov92/LinearApproximation/blob/main/Linear.h
+    template <typename Key, typename Value, std::size_t Size> class LinearApproximation {
+      using Point = struct {
+        Key key;
+        Value value;
+      };
+
+    public:
+      LinearApproximation(const std::array<Point, Size>&& sorted_points) : points {sorted_points} {
+      }
+
+      Value GetValue(Key key) const {
+        if (key <= points[0].key) {
+          return points[0].value;
+        }
+
+        for (std::size_t i = 1; i < Size; i++) {
+          const auto& p = points[i];
+          const auto& p_prev = points[i - 1];
+
+          if (key < p.key) {
+            return p_prev.value + (key - p_prev.key) * (p.value - p_prev.value) / (p.key - p_prev.key);
+          }
+        }
+
+        return points[Size - 1].value;
+      }
+
+    private:
+      std::array<Point, Size> points;
+    };
+  }
+}