Adding offset parameter to the tau isolation

Modification in *byIsolation discriminant. Here offsetRelativeSumPt parameter was introduced to allow better combine absolute and relative isolation. Say, you want to have absolute isolation <3GeV till 60GeV and above it relative <0.1/Pt. To do so, you need move the latter by 30GeV, ie apply cut as <0.1/(Pt-30). It is new configuration possibility which is not being used by any sequence run by default, so no changes are expected.

@veelken, @mbluj