Check the use of deepcopy(rng)

[jschepers]

TODO: Go through the package code and check whether deepcopy(rng) is needed or redundant/not wanted. Try to make it consistent throughout the package e.g. on which level deepcopy is used.

[behinger]

• document in repeatdesign
• make new repeatdesign type-fields: event_order_function::Function = x->x and copy_rng::Bool=false