is_dir failing sometimes

[l-cadamuro]

The helper function is_dir defined in AnalysisHelper.cc sometimes fails upon launching fill_histograms.exe and the code complains about the output folder existing while nothing has been created.
Not sure about the cause and it's typically enough to just relaunch the command.
One thing that could be tried is the filesystem library of C++:

#include <experimental/filesystem>
if (std::experimental::filesystem::is_directory(outputFolder_))

Clearly not critical to be fixed :-)