Calculates the product across choice observations for the same individual.
The function makes a call to `reshape_panel`

and
`replace_na_padding`

prior to taking the
product.

`panel_product(x, n_ct)`

- x
A matrix with the probabilities of the chosen alternative

- n_ct
The number of choice situations per individual

A vector of sequence probabilities. If our model contains inter- individual draws, then this vector is n_id * n_draws