Added REQUIRE-KB

[cheryllium]

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.