Some renaming of types for multivariate normal distributions

- The generic type that can take different covariance types is renamed to ``GenericMultivariateNormal``
- ``MultivariateNormal`` is now alias of ``GenericMultivariateNormal{PDMat}``, which refers to those with full covariance
- introduce ``DiagNormal`` as the alias of ``GenericMultivariateNormal{PDiagMat}``, which refers to those with diagonal covariance
- introduce ``IsoNormal`` as the alias of ``GenericMultivariateNormal{ScalMat}``, which refers to those with isotropic covariance
- introduce a convenient function ``gmvnormal`` to construct normal distributions of different types using simplified syntax