Clarify units in the material description

Following the changes proposed in !597 (merged), the current unit definition suffers from a potential numerical issues. Since g would be defined in terms of GeV, quantities like the material density, i.e. g/cm² would correspond to a huge, i.e. 10^{27}, numerical value within the native units. However, the way densities are used in the material description means they end up being divided again by a factor 1mol \sim 10^{24} resulting in a reasonably numerical value again. By storing e.g. only the effective quantities in the material description would prevent this.