Adding optional PU weighted neutral isolation to tau ID

Adding an option to run weighted neutral isolation for the tau isolation. The very same algorithm has been already implemented by #4131 for general use. As the tau isolation is calculated separately, it was necessary to implement this also within tau ID software.

The default RECO sequence is not affected, no changes expected.

The new option can be utilized via
```
--customise RecoTauTag/Configuration/addWeightedIsolation_cfi.addWeightedIsolation
```

The studies that show that this way of calculating isolation is superior to the previous ways of handling neutral isolation has been presented by Yuta at CPF: https://indico.cern.ch/event/322762/

Fine but you can get rid of more code by isoNeutral_ = isoNeutralWeight_filter; then any use of  calculateWeights_ is not needed any more below. You may say it is a bit more obscure (isoNeutral IS the weighted one) but after all this is the intention for calculateWeights. You also have to fix the print outs (no neutralPt and weighted neutralPt at the same time except if you want to see the init value) and maybe variables used - clean as you like.