Add `inverse` parameter to `convert` and `transform` methods

[gschulze]

The project method for the Proj struct accepts an inverse parameter. No such parameter exists in the transform and transformed methods of the Transform trait. Internally, those methods are calling the convert and convert_array methods on Proj, which also do not have an inverse parameter. However, the proj_trans function, which is called by these conversion methods, accepts a direction parameter which is currently set to PJ_DIRECTION_PJ_FWD.

My suggestion would be to introduce a breaking change and, in doing so, also improve the naming:

• Rename the convert and convert_array methods in Proj to transform and transform_array, and add a boolean ìnverse` parameter.
• Add a boolean inverse parameter to the transform and transformed methods in the Transform trait