Show residual in georeferencer in map units (if possible)

[mhugent]

The following patch shows the residuals in the gcp table in map units where possible and in pixels else. Because interpretation about the gcp errors is easier with map units, so the patch converts pixels->map units for linear transformation and helmert. For the other transformations, conversion to map units is not so straightforward, therefore they are kept in pixels.

[mhugent]

Added improved version of the patch. The transform parameters are now shown in the status bar, fixed update of georef transformation when enabling/disabling gcps in the table, fixed pdf report layout

[mmassing]

Applied in r13552 - thanks, Marco.

I'll leave the ticket open for now, because I'd eventually like to add this functionality for the higher order transform types (linearization via jacobi matrix or numerical approximation of the derivatives), and also a config option to choose the desired units.

I think the latter is useful because reporting residuals in pixel units shows if residuals are in the ballpark of input discretization, whereas map units are useful to better get an estimate of absolute error.