ConfigManager: delay parsing of detectors_file

Currently, the detectors_file parameter is parsed in the constructor of the ConfigManager, which means it is not possible to overwrite it from the CLI using

allpix -c config.conf -o detectors_file=somefile.conf

This MR delays the parsing of the detectors_file key until the detector configurations are actually accessed (a bit similar to the lazy-closing of geometry). This allows to load module options before and to overwrite the key from command line.

A test has been added to specifically test this.