Set seed

[lindeloev]

There should be a seed-argument for all functions involving randomness cf this question (and comment-answer) on StackOverflow: https://stackoverflow.com/questions/73416610/why-is-set-seed-not-working-for-mcp-r-package

Suggested behavior for mcp():

• mcp(..., seed = 42) sets a seed before producing chains "random" numbers, passing them as seeds to rjags chains.
• mcp(..., seed = c(1,2,3)) requires length(seed) == length(chains) and passes these seeds directly to rjags chains.
• mcp(..., seed = NULL) (default) lets rjags set seeds for the chains.

tidy_samples(), fitted(), plot(), etc. should just accept an atomic numeric seed or NULL.