Add collection method to Prefix

[keilw]

While most Prefix implementations use a Java enum, its values() method is only available in the actual implementation. Can we add a method to Prefix that works like SystemOfUnits.getUnits()?

For enum based implementations, it seems rather straightforward via:
return Collections.<Prefix>unmodifiableSet(EnumSet.allOf(MetricPrefix.class));
see https://stackoverflow.com/questions/6528570/converting-from-enumseta-to-setb-when-a-inherits-from-b

Hopefully easier vote on how to call this method?

[keilw]

A) prefixes()

Inspired by the values() of enum (although Prefix implementations are not obliged to use enum)

[keilw]

B) getPrefixes()

Closer to e.g. similar methods in SystemOfUnits.

[keilw]

Actually a static method ended up a better option. While SystemOfUnits returns a Set of units, an instance of Prefix does not have to return all other prefixes. At the moment the SystemOfUnitsService returns the Set given the name of a Prefix enum like "MetricPrefix".