Set random seed when running a simulation

Up to now the ROOT random number generator was always initialised with the
same default value. In some cases this gives strange results when doing
large scale simulations with several jobs.
To solve the problem TRandom is now initialised with the value 0 which uses a
seed value based on the system time with a granularity of 100ns.
This granularity should be good enough to have different seed values even if
several jobs start a similar times on the batch farm.
A setter was added to allow to set the seed value explicitly from a macro,
refs #1395, #2066