Add option to change covariance matrix type for GMM class

This PR adds changes to the `zuko.flows.mixture.GMM` class, which allow the user to change the type of the covariance matrix used for each of the Gaussian components of the mixture. The options added are - `covariance_type`, which allows to change the type of the covariance matrices - `tied` a switch which allows to control if covariance matrices are tied between components - `cov_rank` the rank of the low-rank covariance matrix when `covariance_type` is 'lowrank' Since the construction of the shapes got quite long I moved this part in its own function. Below is an illustration of the effect these different choices have for a mixture of 3 two-dimensional Gaussians.