[ENHANCEMENT] Change the unit of neutron scattering lengths to fm

[MBartkowiakSTFC]

Is your feature request related to a problem? Please describe.
MDANSE has its own database of atom properties. Neutron scattering lengths are given there, but they seem to be given in Angstroms, which is not the usual unit for this property.

Describe the solution you'd like
Most scientists will expect the scattering length to be given in fm and scattering cross-section in barns.
https://www.nist.gov/ncnr/neutron-scattering-lengths-list

Describe alternatives you've considered
N/A

Additional context
N/A

[oerc0122]

I guess the other question (and this is probably another issue in and of itself) is whether MDANSE needs its own database of atom properties, does something like periodictable provide the properties MDANSE needs with lower maintenance and unified interface better than the big json file?

[MBartkowiakSTFC]

The main argument for keeping at least part of the current mechanism is that users sometimes define their own artificial chemical elements to reflect an unusual isotopic mixture, and the "CenterOfMassesTrajectory" job replaces molecules with atom-like objects stored the same way as atoms. It may still be meaningful to use something like periodictable as a starting point, but ultimately we will still need to keep the interface for saving and loading atoms if we want to keep all the functionality that we offer at the moment.

[oerc0122]

Periodictable does support compounds, but I see if these are abnormal physically (i.e. dummy atoms) a backup method might be valuable and complex to implement.