Typescript overrides for derived getters shouldn't also have a setter.

[ikeough]

To represent derived attributes in Typescript, we currently need to include a get and a set, otherwise the setting of the property via the constructor will fail with a warning that the class has only a getter. Of course, we don't want a setter because these attributes are derived. We should only have a getter which calls the corresponding function to return the correct data. This needs a better solution.