Backports of 71x tau developments to SLHC

This branch contains backports developments in the RecoTauTag/* packages done in 71x and mostly already included in CMSSW. The new features are the discriminators with delta Beta PU weighting which have been tested during the Run II preparations and have shown very nice performance. Finally, there is removal of the eta cuts on the tau reconstruction in order to allow reconstruction in more forward regions that are instrumented in upgrade geometries.

@monicava, @veelken