smoothed ship spawn rate function (#553)

## Description:
Fixes #552 
## Please complete the following:

- [ X] I have added screenshots for all UI updates
- [ X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [X ] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

Discord Name: 1brucben

src/core/configuration/DefaultConfig.ts

@@ -251,12 +251,7 @@ export class DefaultConfig implements Config {
     return 10000 + 150 * Math.pow(dist, 1.1);
   }
   tradeShipSpawnRate(numberOfPorts: number): number {
-    if (numberOfPorts <= 3) return 18;
-    if (numberOfPorts <= 5) return 25;
-    if (numberOfPorts <= 8) return 35;
-    if (numberOfPorts <= 10) return 40;
-    if (numberOfPorts <= 12) return 45;
-    return 50;
+    return Math.round(10 * Math.pow(numberOfPorts, 0.6));
   }
 
   unitInfo(type: UnitType): UnitInfo {