Set DM.UseSaveDM to false. Previously existing *.DM files will not affect the exercise

Julio Gutierrezrequested to merge
juliogut/siesta-docs-scf-tutorial-fix:master into master

Set DM.UseSaveDM to false. Any *.DM files in the working directory will be ignored, so tests are not affected in case the user copies the directory from a previous run.

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