DOC: documentation of placement of detectors for CylindricalPET/ECAT is unclear

[KrisThielemans]

I don't see anywhere in the documentation if a "box" is centred around 0,0,0, or one of the corners. I'm assuming it's the former, but it would be great to be sure :-)

box definition is here

Gate/docs/defining_a_geometry.rst

Lines 289 to 295 in 0165845

	| BOX | TRPD |
	+=============================================================================+=================================================================================================+
	| setXLength: Set the length of the box along the X axis | setX1Length: Set half length along X of the plane at -dz position |
	+-----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------+
	| setYLength: Set the length of the box along the Y axis | setY1Length: Set half length along Y of the plane at -dz position |
	+-----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------+
	| setZLength: Set the length of the box along the Z axis | setX2Length: Set half length along X of the plane at +dz position |

The CylindricalPET example gives

Gate/docs/defining_a_system_scanner_ct_pet_spect_optical.rst

Lines 493 to 494 in 0165845

	/gate/cylindricalPET/geometry/setRmax 145 mm
	/gate/cylindricalPET/geometry/setRmin 130 mm

Gate/docs/defining_a_system_scanner_ct_pet_spect_optical.rst

Line 521 in 0165845

/gate/crystal/geometry/setXLength 10. mm

Gate/docs/defining_a_system_scanner_ct_pet_spect_optical.rst

Line 501 in 0165845

/gate/rsector/placement/setTranslation 135 0 0 mm

it seems plausible that the translation specifies a new centre at 135, such that the "left-most" edge is at x - xLength2, which corresponds to Rmin.

It would also be helpful to refer to the autoCenter doc such that the reader easily understands that the whole module is centred around 135,0,0.

Other notes:

• this is better documented for the trapezoid and wedge shapes
• • the same problem exists in GATE 10. https://opengate-python.readthedocs.io/en/master/user_guide/user_guide_reference_volumes.html#opengate.geometry.volumes.BoxVolume