Single covariance matrix in mixtures

[mfasiolo]

Functions dmixn, dmixt, rmixt and rmixn take as argument a list of covariance matrices,
one for each mixture components. It would be useful that if the list contains a single matrix,
or if a matrix rather than a list is supplied, that covariance will be used for all mixture components.
This would be useful, for instance, in multivariate (non-adaptive) kde.