Add functions for pension tables and other utility functions

- mortalityTable.zeroes: Generate a mortality table with all zero transition probabilities
- mortalityTable.once: Generates a (deterministic) mortality table with one transition at a given age
- pensionTables.list: Lists all pension tables (similar to mortalityTables.list)
- pensionTables.load: Load a provided pension table (similar to mortalityTables.load)
- transitionProbabilities: Obtain data.frame containing all transition probabilities for a pension table (given the YOB)
- anwartschaften: First attempt to calculate all "Anwartschaften" for a given pensionTable - WORK IN PROGRESS, NOT FINISHED