[willend]

Mantid's LoadMcStas requires 6 event columns (p x y n id t), so a 'mantid square' monitor binned only in x (or y) wrote 5 columns and Mantid silently loaded no events; 'y bins=1' was the workaround.

• In mantid mode with a square monitor, add the missing x or y coordinate with a single bin spanning the monitor, keeping x before y.
• MANTID_RECTANGULAR_DET now finds the x, y and pixel coordinates by type instead of assuming positions 1, 2 and 4, so 'y ... x' no longer gives a 90-degree rotated IDF.
• mcdisplay-mantid: write xstart/ystart as the first pixel centre, as Mantid expects, instead of the bin edge (half-pixel shift, half the detector size in a 1-bin direction).

Fixes #2178

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