Respect desired unit in variable with expression.

[schymans]

If we define a new variable based on other variables, we want to be able to prescribe the desired units rather than taking the base units derived from the embedded expression. Example:

    class local_definition(Variable):
        """Local definition."""
        unit = joule

        class local_variable(Variable):
            """Local variable."""
            unit = kilogram * meter ** 2 / second ** 2
            default = 2

        expr = 1.5 * local_variable

With the current PR, local_definition.definition.unit == joule instead of kilogram * meter ** 2 / second ** 2

[schymans]

I just found a problem with the new derive_unit(). Need to explore a bit more...

[schymans]

OK, solved the problem and added tests.