Higher level API for constructing simulation entities

[ElliotB256]

I would like to see a high level API for creating entities in the simulation, using something like factory structs with a fluent interface. e.g.:

BeamBuilder()
   .in_direction(Vector3::x)
   .with_gaussian_profile(waist, power)
   .with_sigma_plus_polarisation()
   .for_cooling(Strontium)
   .with_detuning(-32.0)
   .build(&mut world);

The advantages of this are:

• user doesn't need to know components required.
• guarantees safe use/initialization of components.
• prosaic code.
• reduces redundancy for duplicate variables (e.g. wavevector)
• nicer editor integration, e.g. autocomplete/prompting. Helpful for new atomecs users.

Disadvantages:

• more code to maintain.
• multiple ways to define things (though this can be alleviated by funnelling users towards higher-level functionality in an atomecs::prelude::* mod.

Originally posted by @ElliotB256 in #29 (comment)