Add a check if AthenaMP is available before usage in MSM
This MR wraps a small dependence on AthenaMP
in the MultipleStreamManager
in a check on availability before usage (as this module isn't available in the AthAnalysis
project and therefore fails when this code is used therein, e.g. here in the AthAnalysis-based PxAOD Maker).
/cc @mvozak (PAOD developer) and @amete (original dependence)