Add `stress` to `MorsePotential`

Presently many classical potentials implemented in ASE do not calculate stress (exceptions: LennardJones, EMT).

This MR adds stress to MorsePotential in the common formalism of Virial stress.

The test is done using numeric_stress.

I also did:

1. Document MorsePotential a bit more.
2. Refactor MorsePotential (mostly renaming variables to be somehow similar to LennardJones).
3. Refactor numeric_stress mostly to give strain consistently in the Voigt notation.

When I have time, I would like to implement stress also for more practical potentials like EAM.

(EDIT: cf. Original MorsePotential implementation in 3d74358a, smooth cutoff introduction in !2073 (merged))

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.