FFJetmearingTool memory leak bugfix

As the title says, there is a memory leak in the FFJetSmearingTool when reading histograms from TFile and passing them to unique_ptr, in the process a copy is made, and the original instance is not cleaned up (why it is not cleaned up when the TFile is closed is not entirely clear to me. The file appears to own the histograms at first glance).

The drop in memory consumption is quite drastic (~ 500MB cca) because the actual histograms being read are very huge in terms of number of bins -- this is separate issue being followed up with JetEtMiss experts.