Worked on require-kb

Changes made by Cherie Yang <cherie@cmu.edu>

- Wrote REQUIRE-KB, which checks if a KB is already loaded before loading, 
  skipping if it has already been loaded. Most recent changes: 
    - Removed printed commentary. 
    - Added verbose keyword argument. 
- Added REQUIRE-KB to the Scone User's Guide. 
- Rewrote core.lisp to use REQUIRE-KB instead of LOAD-KB.