compute the first derivative of log_lambda likelihood component for Langevin-Hastings
log_lambda_gradient(Mu, Nu, Sampled, Area, N, var.nu)
expected value for all cells
current observed valus (all cells)
Vector giving the cell identities for all sampled cells
Proportional area of each sampled cell that is covered by one or more transects
number of groups in each transect
variance of the overdispersion process
a gradient value