test duration chaining
move sorting from unit collection to duration chain
rename TimeUnitCollection to TimeUnitCollector
move TimeUnitCollector and DurationChain out of TimeUnit module
calculate DurationChain on demand
TimeUnits take no parameters anymore
use separate method to calculate unit
use readable methods
make nano multiplier a constant instead of passing to super as parameter
test TimeUnit's default instance values in own test class
extract calculate method from time unit to superclass
set TimeUnit's multiplier by parameter
test and fix month's rest calculation