Heed

Revisit the handling of material properties and photo absorption cross-sections.

• Add a new class AtomDefs that holds a list of the AtomDef objects (static member), instead of keeping a "logbook" inside the AtomDef class. Same for the molecules. Remove global AtomDef and MoleculeDef objects ("GasLib.h/cpp").
• Add a new class PhotoAbsCSLib that initialises and provides access to the atomic photo absorption cross-section objects.
• Create molecular photo absorption cross-sections within TrackHeed (instead of using global objects).
• Simplify constructors of AtomMix, MoleculeDef, MatterDef, GasDef.
• Simplify the class particle_def which contains the relevant particle properties. Create user-defined particles inside TrackHeed instead of using a global object.