7 2 x lifetime patch

Pull request to fix: hypernews posting - Urgent issue with pythia6 + Tauola++ in 71x and 72x (was Fwd: 720pre3 standard relval samples). 
In the hypernews, it was reported that undecayed taus were causing aborts in on an insert statement. There was only one line that could cause this problem (ie setting the tau lifetime for tauola to a non-zero value). This has been changed back and a substitute calculation of the tau decay length has been added with code to propagate it to the full decay chain.
This code has been tested both on default test configurations (see test dir) and on the relval sample that crashed(however, since the generator depends on random seeds, only a limited test could be performed).