Beampipe Vacuum Material Colour

[Laurie Nevay]

Beampipe vacuum material should be automatically coloured if non-standard - especially if used as something dense. A common solution is to set the ‘vacuum’ material as something like aluminium to represent a window at the end of vacuum. In the visualiser it’s really useful to see this material rather than the default (misleading) transparent vacuum.

In the future, we’ll have a window component, but in either case the visualisation should be better.

Minor addition.

[Laurie Nevay]

This is done now. If over a minimal density then a colour will be determined in the usual way. Here, an example is done with water.