Add more Base Units

[fabgio]

Add new Base Units: Millimetre (Symbol:mm), Centimetre(Symbol:cm) and Kilometre (Symbol:km)
Close issue: #451

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This change is 

[andi-huber]

As far as I know, there are exactly 7 base units in SI. Those form a fixed set of building blocks from which other units are derived.

For Example

Kilogram

public static final Unit<Mass> KILOGRAM = addUnit(new BaseUnit<Mass>("kg", "Kilogram", UnitDimension.MASS), Mass.class);

is the SI base unit for Mass and e.g. Gram is derived from it the following way:

/**
 * The SI derived unit for mass quantities (standard name <code>g</code>). The
 * base unit for mass quantity is {@link #KILOGRAM}.
 */
public static final Unit<Mass> GRAM = addUnit(INSTANCE.units, KILOGRAM.divide(1000), "Gram");

If you derive Millimetre, Centimetre and Kilometre adequately from the base unit Metre that would make your code correct.

That said, adding some so called prefixed units and not others to the Units class might be opinionated. But I have no strong opinion against adding those 3.

[fabgio]

As as I know the divide method provides overloaded signatures for performing division but I do not need any of that. If you don't mind I need the mentioned Base Units.

[fabgio]

I think I solved the issue as:
public static final Unit<Length> MILLIMETRE = MetricPrefix.MILLI(METRE); SimpleUnitFormat.getInstance().label(MILLIMETRE, "mm");