Add truncated priors

Following the discussions in #1591 and #750 (closed), we currently do not have a way to introduce a proper truncated prior. But this is an often requested feature that e.g. @rattoma would like to have. Waiting for the new estimation interface (e.g. #824 and #846) seems to long to wait. My specific proposal would be to add a new token truncated_norm_pdf that uses the third and fourth hyperparameter (which is usually reserved for generalized (beta) distributions) to specify the truncation. I consider this a natural interpretation of these hyperparameters for the normal distribution. @stepan-a What do you think? @rattoma Woud that satisfy your immediate needs? Or do you need a truncated distribution other than the normal?