Add checks to prevent certain non-physical inputs being entered by user

[ltmsyvag]

Rubidium().getEnergy(n=5, l=0, j=-1/2) (as an example) returns an energy when it shouldn't (I guess?), since total angular momentum quantum number j is negative and probably it cannot be negative. From the function definition it seems when j == -1/2, ARC does not grab recorded experimental energy data, but rather just calculate energy using Rydberg equation. Seems there can be a positivity check for j in the case of S state?