Make DefaultNumberSystem exchangeable by using e.g. a System property

[jfitzek]

In order to support Double.NaN and not throw an IllegalArgumentException in that case, we had to overwrite the DefaultNumberSystem and have a different implementation for Double. We can currently set our own NumberSystem by calling the setter method Calculus.setCurrentNumberSystem(new MyOwnNumberSystem());

Since we have multiple applications, we would like to have a central system property for setting the NumberSystem in order not to have this line of code in each application and service that uses si-units.

I would suggest changing the line of code in tech.units.indriya.function.Calculus
private static final String DEFAULT_NUMBER_SYSTEM = "tech.units.indriya.function.DefaultNumberSystem";
to something like:
private static final String DEFAULT_NUMBER_SYSTEM = System.getProperty("si-units.calculus.numberSystem","tech.units.indriya.function.DefaultNumberSystem");