update.z: Samples z
In drvalle1/EcoCluster: Bayesian Clustering using Truncated Stick-Breaking priors

This function samples the cluster assignment for each location (z)

## S3 method for class 'z'
update(
  dat,
  nl,
  n.minus.y,
  phi,
  theta,
  ngroup,
  nloc,
  nspp,
  z,
  a.prior,
  b.prior,
  constant
)

`dat`	this matrix has L rows (locations) and S columns (species) and contains the presence-absence data (i.e., number of times a given species was observed at a given location)
`nl`	this vector has L elements (locations) and contains the number of observation opportunities at each location
`n.minus.y`	matrix with L rows (e.g., locations) and S columns (e.g., species), calculated as matrix(nl,L,S)-dat
`phi`	K x S matrix with the probability of observing each species in each group
`theta`	vector of length K with the probability of each location group
`ngroup`	maximum number of location groups (K)
`nloc`	number of locations (L)
`nspp`	number of species (S)
`z`	vector of size L containing the current cluster assignment for each location
`a.prior`	'a' parameter for prior beta distribution
`b.prior`	'b' parameter for prior beta distribution
`constant`	constant term used to calculate probability for a new group