Better way to define vacuum

[Laurie Nevay]

Currently, we have the options vacuumMaterial and vacuumPressure. vacuumPressure only applies if the material is BDSIM’s pre-defined vacuum, which is a mixture of CHO as per warm sections of the LHC. Just now we need low pressure air for example. If we set vacuumMaterial="G4_AIR" and vacuumPressure=1e-3*bar then you would actually just get G4_AIR at normal 1 atmosphere of pressure. The vacuum pressure option is only used in the initial preparation of that material.

1. we should warn users if we maintain the current behaviour (ie the set vacuum pressure but choose a different material)
2. it should be easier / clearer to define such a vacuum material without having to calculate the density from the pressure manually
3. it’s almost a bug that the material isn’t re-prepared with the right vacuum pressure

A future improvement could be in the use of vacuum pressure histogram or function of S where it is sampled and quantised for each component (both pressure and composition if given multiple partial pressures).