Migrates VolumeBuilders to unique pointers

Hi @jchapman, this is my attempt of migrating VolumeBuilders to unique pointers. I found that the easiest way is to get rid of the RegisterVolumeBuilder and UnregisterVolumeBuilder functionalities which aren't used anywhere else. Please let me know what you think. Part of the fixes for ATLASSIM-4657.

Edited by Miha Muskinja

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