Deprecate old cron function in plagiarism API (was "Problems with plagiarism_plugin class")

There are a couple of things that seem a bit inaccurate in the plagiarism_plugin base class:

1. the class should be marked as abstract as it's intended only to be used as a base for extending
2. the plagiarism_cron() method appears to be named wrongly, based on the calling code in plagiarismlib.php (where the plagiarism_cron() function is assuming a method called "cron" exists and calling it)

Neither of these should be a problem, as long as a plagiarism plugin implements a cron() method, but it would be better if the base class could be sorted out.