Parallel task execution and updates for new Schrodinger

• Execute tasks in stage in parallel. Issue #202
• Use absolute path for custom celpp weeks. Issue #204
• Check that Schrodinger env variable is set before using. Issue #205
• Capture Schrodinger version in weekly outputs. Issue #206
• Save number of submitted targets for each participant to REST API. Issue #207
• Handle and prevent empty PDB files created by Schrodinger. Issue #208