select: (optional) if item indices are supplied, only the specified items are used.
resp: item response on all (or selected) items in the object argument. Can be a vector, a matrix, or a data frame. length(resp) or ncol(resp) must be equal to the number of all (or selected) items.
theta_grid: the theta grid to use as quadrature points. (default = seq(-4, 4, .1))
prior: a prior distribution, a numeric vector for a common prior or a matrix for individualized priors. (default = rep(1 / 81, 81))
reset_prior: used for test_cluster objects. If TRUE, reset the prior distribution for each test object.