Implement custom sort functions for both 'dotted' and 'grouped_by_three' reference formatters

In #112 we implemented configurable formatters for reference numbers.

However, this implementation lacked associated sorters for the respective formats that are needed to properly sort by reference number in `ftw.table`.

This pull request adds a `sorter` method to the `IReferenceNumberFormatter` interface, and supplies two implementations for both 'dotted' and 'grouped_by_three' reference formatters.