ST. LOUIS — Agribusiness giant Bayer is asking its employees to volunteer for severance packages as the company readies for layoffs and navigates a shift to a “new operating model.” The move, announced in an email to workers last week, comes after years of financial turmoil and months of recent descriptions from Bayer of an upcoming push to restructure — and trim — its personnel. It was not immediately clear how many of Bayer’s St. Louis-area jobs could be slashed amid the corporate overhaul of the life sciences conglomerate, nor the timeline for making the cuts. The company said Tuesday that …