Make tree shaking possible

Make tree shaking possible for potprox when it’s used as an ES module.

⚠️ This release introduces breaking changes:

• The module doesn’t provide the default export anymore, so you will need to import it either this way:

  import * as potprox from "./dist/potprox.min.mjs";

  or more selectively (which, BTW, makes tree shaking possible):

  import {Morse, Rydberg} from "./dist/potprox.min.mjs";

• The potprox.utils object was removed. Its methods (points and rSqr) are now the methods of a potential instance.