deprecate update_secondary_args

[sbfnk]

I'm unclear on the precise use case for update_secondary_args() which doesn't feature in any of the example work flows. Perhaps it would be good to think about making it simpler for users to pass model fits as priors to others but using this function requires interaction with create_stan_data() which is more of an internal funtion.

Should it be deprecated, and an issue be opened for the broader question of passing previous estimates as priors?

[sbfnk]

Turns out this is being used by estimate_secondary so a better strategy might be to make this internal only.

[sbfnk]

Closing in favour of broader #593