Fix units in material description and interactions

Move the interaction formulas to the Material module and refactor them into simple function with implementations in a .cpp file.

The material parameters and their units are modified as follows to improve numerical stability:

• Radiation and nuclear interaction length stay as-is and are given as native length units.
• Atomic mass must now be given as relative atomic mass, i.e. a unit-less number. It has been renamed to Ar() be consistent with the usual naming as e.g. here.
• Atomic number is still a unit-less number.
• Density is still given as mass/volume in native units and retains the issues discussed in #608 (closed). However, all computations use the molar electron density in mol / volume which is now provided directly by the Material class. This should be a numerical value closer to one.

For now, I consider this a reasonable trade-off that seems to work and does not require larger changes.

Other changes:

• Streamline Material interface, e.g. removing unused MaterialComposition.
• Compute material constants directly in the Material description.
• Streamline MaterialProperties interface by delegating most information back to Material.
• Streamline MaterialComposition and ElementFraction. They are currently unused but might be again in the future.

Closes #607 (closed) and #608 (closed).