Build Gas Materials by Pressure without Density

[Laurie Nevay]

Add a way of constructing materials such as a gas where only the (mass?) fraction is given (or composition somehow) and a pressure without a density so this is calculated internally to make it easy for the user.

• Most likely add another function to BDSMaterials class to do this that calculates density then calls the general AddMaterial function
• May have to alter the checking of which combination of parameters are provided in BDSMaterials - I presume we warn if some are missing
• Update manual (model_customisation.rst).

Discussed together this would be good to add. This is done just now for the general vacuum material, albeit perhaps with some hard coded numbers instead of entirely CLHEP units.

[Laurie Nevay]

Dredging this up - I think it would be good to do so we have control over vacuum pressures nicely.