ENH: Enable observations consisting of multiple variables
ENH: Make shape_from_value method unused and remove it

Exponential family nodes have initialization from value. The
previous implementation required the node to have shape_from_value
method. The implementation of the initialization was improved
and the method became unused, thus it can be removed from all
classes.
ENH: Update plates only if needed in node constructor
ENH: Add a sketch of Gaussian-Wishart node