This function provides a quick way of producing a bandwidth object that may
be used in the lg_main()-function. The user must specify a bandwidth
joint that is used for all joint bandwidths, and the user may specify
marg, a marginal bandwidth that will be used for all marginal
bandwidths. This is needed if the subsequent analyses use
est_method = "5par_marginals_fixed".
The function must know the dimension of the problem, which is achieved by
either supplying the data set x or the number of variables dim.