Tidy up precipitation (#3023)

Fixes #2953

This is an attempt to fix the issue with precipitation amount and
percentage mixup. I have created a separate
`precipitationPercentage`-variable where the probability of rain can be
stored.

The config options now has the old `showPrecipitationAmount` in addition
to a new setting: `showPrecipitationProbability` (shows the likelihood
of rain).

<details>
  <summary>Examples</summary>
  
  ### Yr

I tested the Yr weather provider for a Norwegian city Bergen that has a
lot of rain. I have removed properties that are irrelevant for this demo
from the config-samples below.

Config:
  ```js
{
	module: "weather",
	config: {
			weatherProvider: "yr",
			type: "current",
			showPrecipitationAmount: true,
			showPrecipitationProbability: true
	}
},
{
	module: "weather",
	config: {
			weatherProvider: "yr",
			type: "hourly",
			showPrecipitationAmount: true,
			showPrecipitationProbability: true
	}
},
{
	module: "weather",
	config: {
			weatherProvider: "yr",
			type: "daily",
			showPrecipitationAmount: true,
			showPrecipitationProbability: true
	}
}
  ```

Result:<br/>
<img width="444" alt="screenshot"
src="https://user-images.githubusercontent.com/34011212/216775423-4e37345c-f915-47e5-8551-7c544ebd24b1.png">

</details>

---------

Co-authored-by: Magnus Marthinsen <magmar@online.no>
Co-authored-by: Veeck <github@veeck.de>

tests/unit/modules/default/weather/weather_utils_spec.js

@@ -0,0 +1,45 @@
+const weather = require("../../../../../modules/default/weather/weatherutils.js");
+
+describe("Weather utils tests", () => {
+	describe("convertPrecipitationUnit tests", () => {
+		it("Should keep value and unit if outputUnit is undefined", () => {
+			const values = [1, 2];
+			const units = ["mm", "cm"];
+
+			for (let i = 0; i < values.length; i++) {
+				var result = weather.convertPrecipitationUnit(values[i], units[i], undefined);
+				expect(result).toBe(`${values[i].toFixed(2)} ${units[i]}`);
+			}
+		});
+
+		it("Should keep value and unit if outputUnit is metric", () => {
+			const values = [1, 2];
+			const units = ["mm", "cm"];
+
+			for (let i = 0; i < values.length; i++) {
+				var result = weather.convertPrecipitationUnit(values[i], units[i], "metric");
+				expect(result).toBe(`${values[i].toFixed(2)} ${units[i]}`);
+			}
+		});
+
+		it("Should use mm unit if input unit is undefined", () => {
+			const values = [1, 2];
+
+			for (let i = 0; i < values.length; i++) {
+				var result = weather.convertPrecipitationUnit(values[i], undefined, "metric");
+				expect(result).toBe(`${values[i].toFixed(2)} mm`);
+			}
+		});
+
+		it("Should convert value and unit if outputUnit is imperial", () => {
+			const values = [1, 2];
+			const units = ["mm", "cm"];
+			const expectedValues = [0.04, 0.79];
+
+			for (let i = 0; i < values.length; i++) {
+				var result = weather.convertPrecipitationUnit(values[i], units[i], "imperial");
+				expect(result).toBe(`${expectedValues[i]} in`);
+			}
+		});
+	});
+});