Nonlinear coordinates - fix error to speed up extraction

[sharoncy]

Gergely pointed out that coordinate extraction is not set up optimally in Nutil, it recalculates the deformation field for each pixel (only needs to be done once). I don't understand the details here, great if @leuat and @Tevemadar could discuss and get this fixed.

[Tevemadar]

A possible modification is that I move the triangulation so it happens immediately when the descriptor file is loaded.

[leuat]

This has been implemented. The CoordinateTransform object is now shared in a map, and will only be initialised on creation.