Fix string representations of Transform class

I stumbled upon this inconsistency in the Transform class' string representations, while doing some debugging output.

From the commit message:

The underlying C-array of floats is a 4x4 matrix, of which only a 3x3 subcomponent is relevant in the context of 2D transformations. The access of its indices have been fixed accordingly. There was also a mix-up of col-major and row-major order.

This fix makes the string representations consistent with the signature of the from_values constructor. Tests ensuring this behaviour have been added.