Migrate from TimesliceMulti* classes to TimesliceAutoSource

New convenience classes have been recently added to the flesnet timeslice IPC interface, see https://github.com/cbm-fles/flesnet/pull/90.

This includes a new class TimesliceAutoSource, which provides a generic timeslice-based input interface. It is a wrapper around the actual source classes and provides convenience functions for flexible initialization of different source classes through locator strings. For details of the new functionality, see the header file documentation.

This merge request introduces the TimesliceAutoSource class to cbmroot. The changes included are minor and are intended to be functionally transparent.