Implement Vertical Offset and slope transformation method

[rouault]

@kbevers I was not sure how to map the latitude and longitude of the "evaluation point" of the method, which is the 'origin' of the inclinated plane, to PROJ string parameters. I've used the classic lon_0 and lat_0 parameters, but not completely sure it is appropriate (I have to re-add lon_0 in the forward method, since generic code in PROJ, substracts it, as it is what is convenient for map projections)

[kbevers]

I was not sure how to map the latitude and longitude of the "evaluation point" of the method, which is the 'origin' of the inclinated plane, to PROJ string parameters. I've used the classic lon_0 and lat_0 parameters, but not completely sure it is appropriate (I have to re-add lon_0 in the forward method, since generic code in PROJ, substracts it, as it is what is convenient for map projections)

I think the use of +lat_0 and +lon_0 fits the bill here. The meaning is of course a little bit different but I think it works alright in this case.

[rhuijben]

Can you update the in c declaration header to include the required arguments of the projection, like we have in the other projection definitions?

In my Proj use I try to automatically create some test scenario's based on those. And it would also be nice for the built-in help from the Proj tools.

[rouault]

Can you update the in c declaration header to include the required arguments

done, although as I mentioned in a commit comment, we haven't done that for transformation operations until now, just map projections