Allow user to specify centroid offset for an initial distribution #970
Description
There are applications for which it is useful to allow the user to specify an initial beam distribution with nonzero mean values of (x,px,y,py,t,pt). (That is, the initial beam centroid in phase space does not coincide with the phase space location of the reference particle.) This includes, e.g. jitter studies.
This would allow the user to provide an initial 6-vector describing the initial centroid offset.