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New:
* formulas now report charge and mass_fraction
* formula parser accepts ions as Yy{+} or Yy[]{+} for isotopes
* support neutron activation calculations
* support xray refraction index and mirror reflectivity
Modified:
* update X-ray scattering tables for Zr
* adjust ion mass for number of electrons
* ions now display as Yy{+} rather than Yy^{+}
* fix formula.natural_density
* fix formula.hill so C,H come first
* fix element.interatomic_distance
* formula(value=...) -> formula(compound=...)