Ensure istep used in new diagnostics is not negative.
David Dickinson
Branch: bugfix/fix_istep_minus_one_issue_46
Branch: 8.0.6-RC
Merged
Merged pull request
Merged in bugfix/fix_istep_minus_one_issue_46 (pull request #377)
Merged in bugfix/fix_istep_minus_one_issue_46 (pull request #377)
Prevents out of bounds errors where we index by istep or istep modulo something.
Intended to fix issue #46