Add option to simulate hard pile-up events / delay the charge drift of some hits by some time

Right now, all charge clouds start drifting at t = 0.
We might want to add the option to pass also a set of `t0`-values (when the hits created the electron-hole pairs), such that some charge clouds drift with a slight time delay, e.g. to simulate hard pile-up.

We could at these values `t0` to `Event`, e.g. allowing for
```julia
evt = Event([CartesianPoint{T}(0,0,0), CartesianPoint{T}(0.01,0,0.01)], [100u"keV", 200u"keV"], [0u"ns", 100u"ns"])
```
and then have the second charge cloud only "appear" after 100 nanoseconds.