io_mesh_atomic, utility panel: extension of the option 'custom data file'

Changes
- All properties of the Principled BSDF shader can now be set in the
  data file for each element.
- Some important properties for Eevee are mentioned as well.

Remember: a custom data file contains the size and material properties
for each element. With this, the sizes and material properties of selected
atoms in an atomic structure can be changed at once. The user can therefore
store her/his own material properties inside this file and use it at any
time for any atomic structures.