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This makes the interface consistent between the indexing options and the
merging options.

These needed to be kept up to date with a call to map_all_peaks every
time anything changed - wavelength, detector position etc.  Not doing so
has already led to bugs in another branch, and probably already causes
problems on this branch.

This patch eliminates the rx,ry,rz completely.  Anyone who needs
reciprocal space coordinates for a peak is now responsible for calling
detgeom_transform_coords themselves.

Its in Angstroms, not nm^-1

This seems to give a more compact and logical layout

This should eventually be replaced by something which behaves a bit like
a GtkFileChooserButton

The GUI asks cell_explorer to load all the streams for a processing run,
without knowing if the jobs have completed (or even started) yet.
Therefore, cell_explorer needs to be a bit more tolerant.

Detector geometry is not used for scaling/post-refinement