DEV: Adds a script to test data saved as msgpacks.

The loader_tool currently facilitates:
- Finding the last date in each source
- Forcing a complete reload of benchmark and treasury.
- Manipulating the msgpacks so that trailing dates are dropped,
  so that the condition for an update can be recreated.

This is intended to make it easier to test the update logic added
by @rday in PR #88