More fields on the sample object

[Peter Peterson]

This is to add the fields to the Material and to the algorithm SetSampleMaterial.

• In SetSampleMaterial, add a field for the mass density which will convert the value to a number density if sample composition is known, and generate an exception if there isn't a composition
• Add a field for the packing fraction. This has valid values 0<frac<=1.
• Add a field for the scattering ratio, (<b>^2+<b^2>)/(<b>^2), to be used in PDF reductions.
• Add a field for the mass

[Nick Draper]

Moved to backlog at the end of Release 3.1

[Nick Draper]

Bulk move to assigned at the introduction of the triage step

[Stuart Campbell]

This ticket has been transferred to github issue ​8857