gradient for negative log likelihood function in zero-inflated Poisson hidden Markov model with covariates, where zero-inflation only happens in state 1
grad_zipnegloglik_cov_cont(parm, y, covariates, M, ntimes, timeindex, udiff)
working parameters
observed series
design matrix of covariates including the intercept
number of hidden states
length of the observed series
vector of observed time points
unique time intervals
gradient for negative log likelihood