make "trigger_paths" a non-dense array
In the recent release of art, we now can set the trigger bits of each given path, BUT the bits sequence has to be contiguous, in order to keep trigger_paths dense. The request is to allow trigger_paths to be non-dense. In this way, we can set arbitrarily the bits to the various paths.
This feature is especially useful for the Mu2e case because when we will be in data-taking mode, the Trigger Menu might change from Run to Run, thus having the possibility to keep the same trigger_bit to the same paths would immensely simplify the bookkeeping and the operations downstream needed to handle/process the data.