Use Immutable AbstractBoxType as type parameter
Use 0-length Array3D instead of Union(Array3D, Void)

This give me more type-stable functions
[doc] End of the Universe module doc
[doc] Reorganise files
[doc] End of the Atom module documentation
[doc] Fix and tweak the LaTeX PDF output
[doc] Better MD steps figure
[doc] Better functions markup
[doc] Start the Potential documentation