This is a generative function. The user chooses one of the process
: "iid", "AR1", "AR12", "MA12", "Nonmixing", "sysdyn", and it
generates the chosen process. These processes are fully described in the paper of
E. Caron, J. Dedecker and B. Michel (2019). Linear regression with stationary errors: the R package slm. arXiv preprint arXiv:1906.06583.
https://arxiv.org/abs/1906.06583.