add new USS rates

Add the new USS contribution rates and update expected test output.

This MR updates the version number to 1.2.1 and so a new tag will be need to be made after merging.

The commits in this MR are structured carefully. The first enables the 2019 NIC tables and fixes up tests. The second changes the way pension rates are specified but keeps the existing rates and ensures that no change in test output occurs. The final commit then adds the new pension rates and fixes up the tests.

Closes #18 (closed)