Cross-section Bias Object Attachment Improvements

[Laurie Nevay]

Need to:

1. document better all the bias options for every part of the model (every part should be able to be biased in some way) explaining the categories better
2. distinguish the air ‘inside’ a magnet outer or container
3. clarify what is mean by “vacuum” and “material” in the context of biasing attachment
4. allow biasing for placements - therefore add ability to categorise volumes by name for placements

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