PanTauAlgs: enable thread-checker and fix warnings

PanTauAlgs modifies (the mass of) the TauJet constituents when building the PFO. To avoid const_cast add a non-const accessor in TauJet_v3. Since ElementLink does not provide non-const access to the pointed to object, take the detour via the container itself.

Note, this is not an EDM change. Just an addition to the interface.