Add the configurable ability to enable ROOT's implicit multi-threading to art.
Starting with ROOT v6_10_00a onward, ROOT is built with TBB and implicit multi-threading support. In order to activate implicit multi-threading at runtime,
gROOT->EnableImplicitMT() must be called, although in order to avoid conflict with our own initialization of TBB the latter must come before the former.
I propose we provide an optional parameter, say,
services.scheduler.enableROOTImplicitMT (defaulting to
false). However, before calling
gROOT->EnableImplicitMT() we may wish to have braces in addition to belt by checking for the presence of the word, "imt" in the result of
gROOT->GetConfigFeatures(), since an attempt to activate IMT without support present will most likely result in an exception.