fix and tests for lost precision in unit conversion

Author: willem.romijn

measure/src/main/kotlin/com/alliander/open/measure/Measure.kt

@@ -47,7 +47,7 @@ data class Measure<U : Units>(val amount: BigDecimal, val units: U) : Comparable
     infix fun <A : U> `in`(other: A): BigDecimal =
         if (units == other)
             amount
-        else (amount * units.ratio).divide(other.ratio, amount.scale() + units.ratio.precision(), RoundingMode.UP)
+        else (amount * units.ratio).divide(other.ratio, amount.scale() + other.ratio.precision(), RoundingMode.UP)
 
     operator fun plus(other: Measure<U>): Measure<U> = baseUnits(
         units,

measure/src/test/kotlin/com/alliander/open/measure/MeasureTest.kt

@@ -218,4 +218,14 @@ class MeasureTest : StringSpec({
 
         result shouldBe expectedResult
     }
+
+    "Unit conversion uses the correct scale" {
+        val energyInJoule = 13500000 * joule
+        val valueInKwh = energyInJoule `in` kiloWattHour
+        valueInKwh.stripTrailingZeros() shouldBe BigDecimal.valueOf(3.75)
+
+        val powerInW = 1.99E7 * watt
+        val valueInMw = powerInW `as` megaWatt
+        valueInMw.amount.stripTrailingZeros() shouldBe BigDecimal.valueOf(19.9)
+    }
 })