Introduce possibility to simulate time-based with constant time between events. Refs #1792.

In time-based simulations, the time between two subsequent events is sampled from an exponential distribution (Poisson process). For debug reasons, it was requested to have the possibility to generate constant time intervals between events - see #1792. This MR introduces the required functionality.

For this, a new enum cbm::sim::TimDist was introduced, which can have the two value Poisson and Uniform. It has to be specified when specifying an input to the digitization run, as can be seen in run_digi.C:

CbmDigitization run;
cbm::sim::TimeDist timeDist = cbm::sim::TimeDist::Uniform;
run.AddInput(0, evntFile, timeDist, eventRate);