Add plugins to dump masses of volumes and perform radiation length scans

Add a few plugins to aid geometry developers:

• one that traverses the volume tree and prints out the capacity, density and mass of each volume in the tree
• one that creates radiation length scans in x vs y, x vs z, or y vs z for an interval in the 3rd cartesian coord
• one that creates radiation length scans in eta vs phi for specified intervals in z
• one that creates VELO-specific radiation length scans in eta vs phi (broken down by material type, before the 1st hit, 2nd hit, etc.)

cc: @mmonk

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• Core Software