partly converted Length unit to an immutable value object
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/org/wahlzeit/astronomy/LengthUnit/LengthFactory.java
	src/org/wahlzeit/astronomy/LengthUnit/LengthUnit.java
	src/org/wahlzeit/astronomy/LengthUnit/impl/AbstractLengthUnit.java
	src/org/wahlzeit/astronomy/LengthUnit/impl/KilometerLength.java
	src/org/wahlzeit/astronomy/LengthUnit/impl/LightYearLength.java
	src/org/wahlzeit/astronomy/LengthUnit/impl/ParsecLength.java