Add --shape option with selectable shape parameters #308

We do not have a way to implement arbitrary shape morphs apart from the user writing their own funcy multiplier function.

To solve, let us implement a --shape option that allows users to select out of a few allowed nanoparticle characteristic functions. Then, there should be a corresponding --shape-parameters that allows the user to put in the starting guesses for the parameters required for that particular shape function.

We will need to add:

• The --shape option and --shape-parameters option.
• Documentation about the shapes available and the corresponding parameters for each shape.
• Error messages that tell the user what parameters are required for their particular shape if they have not chosen the right one.