Add forecast daypart derivation

internal/forecast/thresholds.go

@@ -0,0 +1,20 @@
+package forecast
+
+func DifferenceAtLeast(previous float64, current float64, threshold float64) bool {
+	return abs(current-previous) >= threshold
+}
+
+func CrossesAtOrAbove(previous float64, current float64, threshold float64) bool {
+	return previous < threshold && current >= threshold
+}
+
+func CrossesBelow(previous float64, current float64, threshold float64) bool {
+	return previous >= threshold && current < threshold
+}
+
+func abs(value float64) float64 {
+	if value < 0 {
+		return -value
+	}
+	return value
+}