Remove correlation with z / t in "square" bunch distribution

[Laurie Nevay]

By default, in square bunch distribution, z is calculated from t by assuming a linear relation and using the velocity as c. We patched this by allowing you to specify envelopeZ=0 and noticing that that has been set, we don’t do the correlation.

This distribution is our uniform random one without correlation so should be uncorrelated in all dimensions.

[Laurie Nevay]

Fixed for next version. The default is now to be uncorrelated. I allowed the previous behaviour to be recreated with:

beam, zFromT=1;

which only applies to this distribution.