Initialisation Interface

[nic-barbara]

We should change the way models are initialised to make things more streamlined. For example:

• Pass a function handle to DirectRENParams constructor to use that function as an initialisation
• Also give the option of using default initialisers

We currently have two random initialisations. One using glorot_normal() for most everything, and the other setting implicit params E, F, P to the identity and constructing direct params using a Cholesky factorisation. These can be the defaults.