Get nodes/variables from driver by name

[Riezebos]

Is your feature request related to a problem? Please describe.
This is a minor inconvenience, but currently if I'd want to get a node from the driver by name, I'd have to do:

node = next(n for n in dr.list_available_variables() if node.name == "the_node_i_want")

Describe the solution you'd like
A simple way to get a node by name, for example:

node = dr.get_variable("the_node_i_want")

or

node = dr.variables["the_node_i_want"]

The bottom option seems slightly nicer as in an interactive session dr.variables could be called on its own first to have a look at all the names and then called with the correct name.

Additional context
I looked at the code and a potential solution for the above would be to add the following to the Driver class:

    @functools.cached_property
    def variables(self) -> dict[str, Variable]:
        """Returns a dictionary of all the variables (HamiltonNodes) in the graph."""
        return {k: Variable.from_node(v) for k, v in self.graph.nodes.items()}

or

    def get_variable(self, name: str) -> Variable:
        """Returns a variable (HamiltonNode) by name."""
        return Variable.from_node(self.graph.nodes[name])

[zilto]

Linking this relevant issue on have a __getitem__ on the Driver or Driver.graph: #836

And this PR that added __getitem__ on the HamiltonGraph: #919

[elijahbenizzy]

Adding on this -- get the HamiltonGraph object from the driver as well