Allow customisation of default ACTS logger (was ACTS-271)

Original author Christian Gumpert @cgumpert

ACTS provides a convenience function for creating a {{Logger}} object:

{code} std::unique_ptrActs::Logger Acts::getDefaultLogger(...) {code}

The {{Logger}} class in ACTS is designed to be able to work with different backends for outputting the log messages. This allows to integrate the ACTS Logger into already existing logging mechanisms in other frameworks (e.g. Gaudi) by specifying a custom {{OutputPrintPolicy}}. Unfortunately, the {{getDefaultLogger}} method is used internally in several places by ACTS methods (e.g. geometry converters and alike). Therefore, it would be great to provide a mechanism for overriding the implementation of this function by end-users (e.g. to pipe the ACTS log messages through the logging system of the framework).