update allowing the merge of rings with overlap in z direction

The update allows to regulate the number of subvolumes by adjusting the minimal radial gap required for ring layers to be separated into different subvolumes ( property "MinimalRadialGapForVolumeSplit" ). The default value (2mm) ensures the number of material layers is the same for ATLAS-P2-ITK-17-07-00 and 17-08-00, which means the same version of material maps can be used for both ( the aim is to have the same input material in GeoModel input for these 2 layout versions in order to test the new pixel ID scheme). The code allows to merge simple 1D (phi binning) disc layers into 1D1D (R,phi binning) disc layer. Merging of 2D layers not support for the moment.