Decouple mock ID from random seed used in the different steps #1
Description
By default, we use the same integer (0-99) to identify a particular mock realisation, and to specify the random seed to be used in the different steps: in the making of the quasar catalog (the downsampling and magnitude assignments) and in quickquasars (to assign quasar continua, random BALs and to add instrumental noise).
However, sometimes we might be interested in using different random seeds for a particular mock realisation (!%$Z!&!), in which case it would be convenient if the book-keeper didn't use the same variable seed to refer to both.
We could for instance, have two names mockid and seed, and if seed was not specified then we can set seed=mockid to recover the current settings.