Add grouping option for pixel and tube
Adds the possibility to select and group individual pixel and tube. HOWEVER due to these selections being dependent of the detector, contrary to every other shape used for masking etc, the selection cannot be printed on screen, as it would require to create a shape that follows the detector if it is moved and is rendered using the correct perspective. Moreover, contrary to what happens with masking and ROIing, there are two options for grouping, which means the operation cannot be applied immediatly on click in this case. Since this means that a) the user should input after doing their selection and b) this selection cannot be shown to the user on the instrument view, the compromise solution was to only take into account and store the last tube or pixel selected for grouping. In other words, the user can only sum or extract the last pixel / tube they selected.