Creating NDVariableArray is slow

[antoine-dedieu]

We are currently adding the variables one at a time via:

names_to_variables = collections.OrderedDict()
for name in itertools.product(*[list(range(k)) for k in self.shape]):
    names_to_variables[name] = nodes.Variable(self.num_states)

Dropping the Variable object to create a numpy array can be 100x faster