Define reverse transformations

[harry093]

A transformation object, gda94_to_gda2020, has been defined to transform from GDA94 to GDA2020. To transform the other way, the negative of the transformation object can be used, i.e., -gda94_to_gda2020. This means people need to look up (or remember) that gda94_to_gda2020 is a defined transformation object and that gda2020_to_gda94 is not. All transofrmation objects should have their reverse defined. For example,

gda94_to_gda2020 = Transformation(...)
gda2020_to_gda94 = -gda94_to_gda2020