update function

[joelpick]

would be useful to have a function that allows minor updates to parameter list, rather than complete respecification, e.g.
update(group, parameter, values)

[joelpick]

give updated parts of the parameter list - gets merged and then passed back to simulate_population? update(squid, parameters). Maybe anything new that gets passed to it is replaced, for example data_structure

[joelpick]

would be good if sampling could be updated without changing simulated data