keep checksums in main repo not models repo

having them in the models repo has added an unnecessary layer
of hassle to maintaining them.  I think it was a mistake (mine).
Merge remote-tracking branch 'origin/5.0.x'
Merge remote-tracking branch 'origin/master' into core

Conflicts:
	headless/src/main/org/nlogo/headless/ChecksumsAndPreviews.scala
	headless/src/test/org/nlogo/headless/TestChecksums.scala