Inconsistent behaviour of `get_units` with References

[andrewlee94]

When using the get_units method on a Reference to a component with units, the output is not the same as if the get_units method was applied to the component itself. A simple example is shown below:

m = ConcreteModel()
m.var = Var([1, 2, 3, 4], units=units.m)
m.ref = Reference(m.var[:])

print(units.get_units(m.var))
>>> m

print(units.get_units(m.ref))
>>> None

Note that if applied to elements of the Reference, the result is correct (i.e. print(units.get_units(m.ref[1]) returns m).

In this case, the get_units method should probably return the correct value or raise an Exception. Otherwise, this could create confusion for users.