Added temp comment

It concerns the expected behavior of interface implementers. Later will
be replaced with proper XML comment.
Made AggregateUnitConverter a provider

Now AggregateUnitConverter<TValue> implements
IUnitConversionProvider<TValue>. Later this class will be separated into
two: AggregateUnitConversionProvider<TValue> and
AggregateUnitConverter<TValue>.